Sean K. Reynolds said:
Here's where things get totally crazy.
First, if undead could sense life force, that would be a HUGE advantage. They could ignore invisibility, mirror image (which, mind you, is a figment, and as explained above produces a real image that creatures can actually see, not a mental trick), blindness effects***, blur, displacement, magical darkness, most concealment effects, know when a creature is standing around a corner (unless this "lifesense" requires a line of effect to the living target), spot hiding creatures automatically ... and on and on. Clearly the lowly skeleton is a much greater threat because of this! What evil necromancer or cleric in his right mind wouldn't keep some skeletons on hand to alert his living minions of invisibles and such in the area! We must increase the CRs of all undead immediately! After all, this lifesense operates like a combination of deathwatch, see invisibility, and true seeing!
But wait ... again we run into the problem that none of the D&D books say that undead ignore invisibility and so on. And this lifesense brings up a lot of questions. Can they sense nonliving things that aren't creatures, like walls? Deathwatch says they can recognize nonliving creatures such as constructs but doesn't say anything about recognizing nonliving objects, presumably because the default assumption of a human caster can already sense that a floor exists, but the lifesense argument cannot use "undead have human senses" as an assumption because the lifesense conclusion is trying to explain why undead do not have human senses.**** It would make it a lot harder for undead to get around if their only perception of their environment was through living or undead landmarks. Heck, if they can't sense objects, undead monks wouldn't be able to use Deflect Arrows since they couldn't perceive the arrow in flight or even as it was leaving the enemy's bow. Same goes for Improved Disarm and Sunder ... how can you knock a weapon out of someone's hand or attack the weapon if you can't actually sense the weapon in the first place? Heck, how can undead pick up weapons and use them in combat? They can't see a scabbard to attach to a belt (which they also can't see), and they can't see the sword in the scabbard. Undead must walk around with their weapons out all the time because if they ever put down their weapon they'll never be able to find it again! And good luck using magic items ... a wounded undead trying to dig a potion of inflict critical wounds (which it can't see) out of its pouch (which it can't see) so it can heal itself won't be able to read the label on the potion (since the label and paper are not living)!
(As an aside, notice the sample vampire in the MM has the Blind-Fight feat. Why would it ever need this feat if it could sense invisible creatures and operate in total darkness? You could argue that it had that feat while it was still alive and just has to "eat" the choice of a bad feat that does nothing for its undead self (just like the Diehard feat would do it no good, since as an undead it can never go to negative hit points). But why would WotC deliberately give a bad feat selection into a creature in the MM? Wouldn't that make the creature weaker than its CR would indicate? You don't see them giving Improved Unarmed Strike feat to longsword-wielding humanoids--which is just as innefectual (and dumb) a choice as the lifesensing vampire with the useless (to it) Blind-Fight feat. Similarly, the sample elite vampire has Blind-Fight and Improved Disarm, which as I point out about is problematic for the undead that can't sense nonliving things.
So maybe they can sense objects walls and floors and such just as well as a human can ... but if you're giving them that, why add in on top of that a special and superior sense that isn't mentioned in the books? If the whole point of lifesense is that undead don't see things like humans do, why turn around and say that they do see things like humans do and then slap some goodies on top of that--goodies that aren't mentioned in the core books?
Can you hear with lifesense? Taste? Touch? Smell? No answers. Wait, undead have to be able to hear, since their creator can give them verbal commands. So is that a quirk that they can hear only because it comes from a living creature? Why would this sense differentiate between sound waves made by a living creature and those that aren't. I guess lifesense lets you hear. Probably like a human. Which again makes me wonder why they need a special sense if they're already using human senses.
What about energy? Acid, cold, electricity, fire, and sonic aren't objects. Can lifesense detect a wall of fire? Can it detect heat at all? Spells?
And what range is this ability? 60 feet? 120 feet? As far as a human can see? This ability isn't defined anywhere. If it has a limited range, then the living should have long ago figured out what that range and exploited it. "An army of undead is on the way? Fear not, just stay 200 feet back and pepper them with arrows, they won't be able to sense where we are."
And let's talk a bit about animate dead. This is a 3rd-level cleric spell that not only crams negative energy into a body, but it gives that undead body this super-powerful lifesense ability! And the duration is instantaneous, which means it can't be dispelled. How come some smart spellcaster hasn't taken that spell apart to find the "give target lifesense" code, strip out the "cram in negative energy" part, and end up with a lower-level spell (since it does less than the original) that gives the target creature instantaneous-duration lifesense? My answer is that there is no lifesense component, that the default senses of any creature are human-normal (in fact, I'd let you create a human senses spell that gave the target human-level senses instead of its own normal senses, but for most creatures that would be pointless). Some people might argue, "no, see, the lifesense comes as part of the natural abilities of the undead life force you're drawing from the Negative Energy Plane." Ok, I can see that, but that still doesn't mean that someone couldn't make a spell called summon a little undead spirit to wrap around my head and relay to me what it senses, which would have the same effect as the give me lifesense spell. Yet no such spell exists.
It's easier and simpler to let undead have normal human senses rather than make assumptions about an undefined yet omnipresent-in-every-undead lifesense ability that you have to keep redifining and clarifying when it's never mentioned in the books in the first place.
...And they don't have lifesense, dagnabbit.
