It doesn't use nearby shadows to provide concealment - the shadowdancer doesn't need concealment - she can hide while being observed.
Yet the Shadowdancer "cannot hide in her own shadow". The implication of this statement is that the Shadowdancer is hiding in other shadows. This seems quite consistent with the statement shadowdancers "weave together the shadows to become half-seen artists of deception". All of this comes straight from the rules.
You want versimilitude? How about this, the HiPS ability lets the shadowdancer become shadow - thus darkvision sees right through it and her, as if she wasn't there. Now it fits your vision of the fluff text and satisfies the RAW.
It seems much further away from the rules quotes above. As well, I would generally expect shadows to be incorporeal. More to the point, normal sight can see shadows, so hiding in plain sight by becoming a shadow like that one 10' away seems unlikely to make me difficult to locate.
Since rangers have HiPS (Ex), is he using shadowstuff to make him invisible? Answer: no. I know the difference between HiPS (Ex) vs. (Su), but it isn't completely non-magic for one, extreme magic for the other, yet called the same thing.
I find it unfortunate they are called the same thing. One is Extraordinary (ie not at all magical) while the other is Supernatural (ie a magical ability). The Ranger can hide while being observed, but is not relieved of the need of "anything to actually hide behind" like the Shadowdancer. Two different abilities with the same name, one completely non-magical (extraordinary abilities lack any magical basis), the other magical.
In a dense forest where the canopy filters much sunlight and shadows abound, the Ranger whose favoured terrain includes forests and the Shadowdancer can both mysteriously disappear from view before an observer's very eyes (Hide in Plain Sight). If we place the area under an anti-magic field, the Ranger still fades into the forest, but the Shadowdancer does not blend into the shadows, rather remaining perfectly detectable.
This is true enough. However, it has no bearing whatsoever on a rules discussion. You wanna know how the mechanics of HiPS work at the table? Read the rules, apply them. You wanna know how HiPS works in the gaming world? Make something up, the (game) world is your canvas. Please don't mix the two, the result is never satisfactory IME.
The game mechanics do not and should not exist in a vacuum. They simulate an aspect of the game world. The Barbarian's foe does not take massive damage because he rolled a 20 and confirmed a critical. He takes massive damage because the barbarian has exploited a weakness in his opponent's defenses, striking at a weak point in his armor to cleave deeply into the vulnerable flesh beneath.
Characters in my games, at least, (as a player or GM) are not wearing bracelets with multicolored numbered polyhedrons which rotate as they act to determine their success or failure. The dice are a metagame construct that arbitrates randomness within the game world. The rules apply mechanical resolution to activities and abilities within the game rules. They are not simply mechanics standing in isolation.
Bull. Sorry to be so direct
There's nothing wrong with being direct. No apology needed.
but what makes you undetectable when you use Stealth (without HiPS!) is not a lack of clanging a hammer loudly on your shield, nor is it a lack of Darkvision on an enemy's part (with HiPS). What makes you undetectable is the simple fact that your Stealth check beat everybody else's Perception check.
These are mechanical simulations. The stealth bonus simulates the skill, experience, training and possibly even magical abilities enhancing the individual's ability to remain undetected. The choice of, say, a full move makes this more difficult and invokes a penalty. The dice evaluate the random aspects of the Stealth-user's success in locating and exploiting a suitable location, or path of movement, or both, including determining the level to which a suitable location/path even exists (a 1 may indicate a squeaky floorboard, while a 20 could indicate noticing that particular board is not level with the others, and should be avoided lest it shift and make a sound). The perception bonus simulates the level of attention the individual pays to his surroundings, the keenness of his sense and his ability to process the subtle cues offered by his environment (the absent-minded wizard with his head in the clouds as he performs spell research in his head is not paying nearly as much attention as the sharp-eyed ranger, always alert for subtle shifts in the environment that may indicate an unknown danger) while the random roll simulates the luck factor (the Ranger was watching the rear, but the wizard just happened to look straight at the concealed opponent's location). Maybe the Ranger is so good that he still perceives the concealed opponent, realizing that squirrel running past must have been disturbed by something, and maybe the stealth user is so skilled that, even looking him straight in the eye, the wizard fails to perceive he is there, so skillfully does he blend with the foliage.
All of this is simulated by the die rolls, but also affected by the environment. It's a lot tougher to hide in the foliage in mid-winter, when all the leaves are gone. And if that Wizard, for all his distraction, is also focusing forward on his Detect Magic spell, all the Stealth in the world won't prevent him from perceiving the aura of that magical dagger in the hidden character's boot. Of course, he still doesn't know much about the magic, and may well assume, even after a Spellcraft roll identifies it as a +2 Elfsbane weapon, that it sits on a tree root, not that it sits in the boot of a carefully concealed enemy who has watched the wizard focus on him for a few seconds with growing concern, and is Readied to attack him, should he move closer.
The HiPS ability modifies the conditions under which you can use Stealth at all. The darkvision ability modifies your enemy's conditions under which he can use Perception, and how well he can use it.
And both interact to create the game reality. Being better able to use perception emhances one's ability to detect a stealthy person. Being able to see through shadows makes the use of shadows as concealment much less viable, where the foliage would confuse dwarf and human equally.
Clanging a hammer might be an example of ex-post-explanation of why a Stealth check failed really badly. Similarly, the enemy hearing your heartbeat might be an ex-post-explanation of his extremely high roll on Perception, coupled with a good skill mod. But don't turn the whole thing on its head.
I don't see myself telling the fellow who rolled a 1 on his stealth roll that he suddenly realizes banging out a tune on his shield with his hammer to accompany his bawdy tavern song may have been a poor approach to sneaking past the guard post. Certainly, it could mean his shield struck a wall, or his swinging hammer clattered against his armor.
I do not, however, care that the character has a +20 bonus to Stealth from being invisible (or even +40 for being stationary) if he tells me "and I am beating out a steady rhythm on my shield". Some actions make it impossible to be stealthy. These include attacking, running and charging. The fact that beating out a clanging rhythm on your shield while singing loudly and out of tune is not included on that list is easily and properly rectified by common sense.
The lack of a more thorough discussion and consideration in the rules as to precisely how the Shadowdancer uses nearby shadows through HiPS, and the implications of attempting to use shadows to hide from a being who can see through shadows as clearly as if they were brightly lit requires a less certain assessment of the combination of effects, which is an unfortunate issue which, perhaps, the rules writers should have better considered.
While leaving this to GM and player judgement is fine, this seems an issue at least as in need of a formal ruling as "you cannot Create Water inside a person's lungs" or "his left eye and his right eye are not valid targets for your two Magic Missiles". Regular discussions of the issue clarify the desirability of a formal and explained ruling.
Then make something up to fit your bill while staying within the rules. It's not really difficult most of the time (ridiculous TO stuff like the Jumplomancer excepted, of course). But when you do it the other way round, you're not using D&D's ruleset anymore. You're making up houserules based on what you see fit. Which can be fine, but can also needlessly gimp players or playstyles, create imbalance where none existed before, or lead to more complicated adjudicatin down the road. Not that this is necessarily the case, just sayin'.
I don't believe ruling on the interaction of shadows to hide and Darkvision constitutes a departure from the rules, and one man's "ridiculous" is another's "high fantasy". The Shadowdancer's player may feel his character is inequitably treated by a ruling that his shadows cannot aid him in hiding from creatures with Darkvision. Equally, the opposite ruling may lead the player who selected a race with Darkvision, purchased a magic item to provide same, or invested his character resources in the Darkvision spell that his character has been unfairly prejudiced.
You can read the ability whichever way you like. But there's a rules-y way to read it, and that one's pretty clear-cut where the actual game mechanics are concerned. Adding stuff based on fluff is coming up with houserules (and in this case, gimping a not-so-strong class further).
As I look to this, I am forced to fall back on the fact that HiPS does not mean "the character uses shadowstuff to become invisible as the spell", but rather "the character can use stealth despite being observed, and with no cover or concealment save the nearby shadows". He must still use the Stealth skill. So, if I assume we have two characters, one hiding in the shadows of that dim light within 10', and a second, a Shadowdancer, standing in the midst of stronger light, but permitted to use Stealth despite that, and being under observation, I have to ask how stealth should work - for both users - in this situation when an opponent, say a Dwarf, has Darkvision.
If I would deny the non-Shadowdancer any ability to use Stealth because he is using shadows that don't impede the Dwarf's ability to perceive him, then the Stealth skill should work the same for the Shadowdancer - his ability permits him to use the nearby shadows as concealment. If, instead, I would give the Dwarf a bonus to perceive the fellow in the shadows, then that same bonus should apply to the Shadowdancer who is using the same shadows to hide from the same person.
As to whether the Shadowdancer is "unfairly gimped", the relative strength and weakness of the various classes is a subjective matter of opinion. I would certainly suggest that, if the player feels this ruling unfairly prejudices his character, the option of retroactively changing his character, or creating a new one, would be appropriate. It's not like the description clearly said "darkvision will prevent this ability from functioning". Similarly, though, the Wizard who invested in the Darkvision spell, and is now frustrated that it does not allow him to penetrate shadows to see those using them to enhance Stealth, should be allowed to retroactively alter his choice if I ruled that Darkvision does not assist in this manner, whether selecting a different spell for his book, or even choosing instead to cast Glitterdust instead of Darkvision.
An interesting question - if, right after the Shadowdancer uses HiPS, someone else creates an Anti-Magic Field, do we assume the Shadowdancer immediately pops back into view (his HiPS being cancelled), or do we assume that he remains hidden (he's now using Stealth, not HiPS)? I'd say if he is still in the bright light area, his shadows fade away as his ability to command them dissipates (ie his ability to use stealth in that location is suspended), however if he had also moved into a dimly lit area, his non-magical stealth is the only thing he is using to hide. If I assumed that his HiPS adds magic to his stealth, I guess I would have to interpret that he becomes visible while the other fellow remains shrouded in the non-magical shadows.
I repeat, make something up. If you really like a PrC's mechanic but not its fluff, change the fluff and keep the PrC. If a game mechanic has no rhyme or reason, change the rhyme or reason, not the mechanic. Fluff is mutable in a magical world far removed from real-world physics, which moreover makes heavy use of literary and pop culture tropes, and also wants the players to feel like heroes.
This is a gamist approach, and the simple use of the term "fluff" implies that only the mechanics matter, not what the mechanics attempt to simulate. In my game, the new "fluff" would also indicate strengths and weaknesses to the ability which may not be crystallized in a two line summary of mechanics. A Shadowdancer is not a collection of mechanics. It is "A mysterious adventurer who walks the boundaries between the real world and the realm of shadows, and who can command shadows to do her bidding." If that does not fit in my game, there will be no Shadowdancers. If it does, then their abilities will be consistent with that description. And a different class, with the same mechanical ability which works in a different way, might well see different strengths and shortcomings to their version of the ability.
Nothing says it can. There's no rules for what you can't do when your dead, either. The one principle that all D&D rules are founded upon is that they must positively say what is possible. Everything not mentioned is impossible.
We now cross into the realm of rules lawyering. Let us assume we have a Magic Apple. It is enchanted such that the Princess of the Realm will be unable to resist its succulent charms, and such that one bite of the Apple will slay her irrevocably. To protect the Princess, the Human Paladin decides that he will eat the apple. It only slays a specific Princess, so he's safe. Great plan.
But the GM says "Show me in the rules where it says a Human Paladin can eat an apple. If the rules do not mention it, it is impossible." A completely unreasonable interpretation, but 100% consistent with your ruling that all of the rules "must positively say what is possible. Everything not mentioned is impossible." If that were the case, why would Create Water need to say it cannot be created in a living creature. It would be impossible, as it is not mentioned. Oddly, the spell does not state the created water can be created within one or more waterskins - is this also impossible? Magic Missile states that "specific parts of a creature cannot be singled out" - why? If it did not say they can, it would be impossible, wouldn't it?
For that matter, it does not say "stealth can be used to hide from Darkvision in shadows", nor does it say "darkvision users can detect those using Stealth in an area of dim light or darkness", only that "It does not allow characters to see anything that they could not see otherwise". Can they see someone using stealth otherwise? It seems odd, actually, that stealth has no bonuses/penalties related to lighting conditions.
That said, in the case of the HiPS ability I agree with you that an area of total darkness should suffice to use it.
Yet by your own reasoning, the ability only says "As long as she is within 10 feet of an area of dim light", not that it can be used if there is no light within 10 feet. Does that not make it impossible? I would have to accept an area of total darkness if I also accept the Shadowdancer's HiPS uses nearby shadows to conceal himself. If I simply assume that the Shadowdancer can use stealth within 10' of dim light, with no assessment of why, in game, then I think I have to enforce "no logic applies - either there is dim light within 10' or there is not".
I have addressed the Ranger HiPS (Ex) ability upthread. The fact that it's called the same as the Shadowdancer ability is bad editing on the designers' part.
Absolutely agree - especially given the two appear in the same book.
A Ranger still needs cover or concealment to hide behind - and if he tries to hide from a creature with darkvision in an area of dim light, he will auto-fail, which the Shadowdancer will not.
Now, I largely concur with the ranger aspect, although I don't see this written anywhere (didn't kill myself looking though). I would have to consider the fact that a person can go unnoticed in a corner of a room, even without dim light or something to hide behind, so perhaps that stealth roll remains valid, perhaps assessing some bonus for the shadows which does not apply against the Darkvision users. As I look at Stealth, the "cover and concealment" discussion seems to be directed at removing oneself from direct observation, and not at saying cover or concealment are mandatory for use of stealth. Interpreted that way, the Shadowdancer ability only permits the Shadowdancer to use nearby shadows to create the concealment needed to override direct observation.
"Normally, you make a Stealth check as part of movement, so it doesn't take a separate action." - does the Shadowdancer have to move, or can he stand and be swallowed by the shadows? I'd say the latter, but this is based on "the fluff" - there's no movement discussion in the HiPS writeup, so back to Stealth, which is less than 100% clear. If he does not move, does he need to use an action to HiPS/use stealth? The skill description suggests he does, as it implies that Stealth requires no discrete action only when combined with movement.
No matter how detailed the rules, the GM/players will need to make interpretations. My model makes those interpretations based on both the concept and the mechanics of the abilities, not on a purely mechanistic view. The mechanistic view probably has to say "cannot HiPS in total darkness; must use Stealth with a Move action; Darkvision makes no difference" The Conceptual view probably means HiPS loses on Darkvision, but it wins on the "darkness" and probably the "move action" aspects.