Are you serious? Have you looked at a 4e stat block?
It has defences. Hit points. Saves. Action points (which for players can also be used in skill challenges, but can't be used in that way by NPCs/monsters because in a skill challenge the GM does not declare actions). Attacks. Movement abilities demarcated in squares.
It's a suite of resources for adjudicating combat.
Here is part of the "devil" entry in the MM (p 60): "Devils torment and consume captured souls to fuel the mightiest of their infernal works, including evil constructs and terrible invocations." Do you really think that, by mistake or editorial fiat, the torment and consume captures soul ability, and the construct infernal work and terrible invocation powers, were left off the devil statblocks?
Those sorts of abilities are not matters for combat resolution. Hence they are not part of statblocks.
And conversely: two of the example skill challenge ideas in the DMG are social encounters (Negotiating with the Duke, and Interrogating the Prisoner). Neither involves reference to any NPC stat block; each involves only the players making checks, against DCs set by the GM not by reference to a stat block but by reference to the DC-by-level table.
The DMG (p 36) says
Before a combat encounter begins, you should have some information at hand. . . . [including s]tatistics for the opponents in the encounter.
Page 72 of the DMG, describing skill challenges under the chapter heading "Noncombat encounters" doesn't contain that advice (because you don't need NPC/monster stat blocks to run a skill challenge). Instead, it says
[indent[The difference between a combat challenge and a skill challenge isn’t the presence or absence of physical risk, nor the presence or absence of attack rolls and damage rolls and power use. The difference is in how the encounter treats PC actions.[/indent]
It then goes on to explain that difference in resolution procedures: that only the players make checks; that the DCs for those checks are set from a chart (not from a stat block); that successes and failures on the checks are tracked, and when certain thresholds are reached in respect of one or the other the challenge is done.
If the GM wants the matter to be resolved by a single ritual, or a single Nature check (probably more plausible for a paragon druid than a 1st level wizard), of course that's his/her prerogative. From the DMG, p 72:
It’s not a skill challenge every time you call for a skill check. When an obstacle takes only one roll to resolve, it’s not a
challenge. One Diplomacy check to haggle with the merchant, one Athletics check to climb out of the pit trap, one Religion check to figure out whose sacred tome contains the parable - none of these constitutes a skill challenge.
challenge. One Diplomacy check to haggle with the merchant, one Athletics check to climb out of the pit trap, one Religion check to figure out whose sacred tome contains the parable - none of these constitutes a skill challenge.
I'm assuming that the dragon's corruption of the water is a matter of some significance in the game, and hence that reversing and preventing it into the future is going to involve more than a simple check. If it's just something being done with a purify water ritual, then all that is at stake is the amount of components consumed.
Also from the DMG, p 74:
It’s also a good idea to think about other options the characters might exercise and how these might influence the course of the challenge. Characters might have access to utility powers or rituals that can help them. These might allow special uses of skills, perhaps with a bonus. Rituals in particular might grant an automatic success or remove failures from the running total.
The use of a Purify Water ritual is the sort of thing that would contribute to a ritual to cleanse a pool corrupted by a black dragon. But the volume of corrupted water is going to depend on the size of the pool, not on the amount of water the black dragon can corrupt per day: given the tendency of contaminants to diffuse through water, either the whole pool is going to need cleansing or none of it is. The size of the pool is something for the GM to decide, and is obviously relevant to the role that Purify Water can play (eg if it's a small pond and the caster can cleanse all of it, then that looks like an automatic success to me, and subsequent checks would be aimed at ensuring no repetition of the contamination by the dragon; if it's a big pond and the caster can only clean a little part of it, then that would be the sort of thing to give someone else a +2 on their next check, eg to summon or encourage water spirits to diffuse from the purified part into the rest of the pond, or as a focus for a prayer to extend the purity through the rest of the pond, or even as a safe part of the water for someone to stand in while they dig a trench or a sump for the contaminated water to drain into).
In 4e, there are no mechanics for how much water a dragon can corrupt. Look at the black dragon entry - they're not there. Where do you think they are hidden?
Also, as I've said, the answer to the question "How much water do we need to cleanse" is found by looking to the size of the pond, not the rate at which the dragon can introduce contaminants.
As for "tunnel vision", I am just restating the basic principles of the game (PHB pp 258-59):
Encounters are where the action of the D&D game takes place, whether the encounter is a life-or-death battle against monstrous foes, a high-stakes negotiation with a duke and his vizier, or a death-defying climb up the Cliffs of Desolation.
. . .
Two kinds of encounters occur in most D&D adventures: combat and noncombat encounters. . . . Noncombat encounters focus on skills, utility powers, and your own wits (not your character’s), although sometimes attack powers can come in handy as well. Such encounters include dealing with traps and hazards, solving puzzles, and a broad category of situations called skill challenges.
. . .
Two kinds of encounters occur in most D&D adventures: combat and noncombat encounters. . . . Noncombat encounters focus on skills, utility powers, and your own wits (not your character’s), although sometimes attack powers can come in handy as well. Such encounters include dealing with traps and hazards, solving puzzles, and a broad category of situations called skill challenges.
The basic structure of the game, and its resolution system, is not kept secret! It's laid out - corresponding to this passage in the PHB are the two chapters of the DMG - "combat encounters" and "noncombat encounters" that I've already mentioned.
There is also another component to the game - exploration - which is described in the PHB (pp 9-9):
Between encounters, your characters explore the world. You make decisions about which way your character travels and what he or she tries to do next. Exploration is the give-and-take of you telling the DM what you want your character to do, and the DM telling you what happens when your character does it. . . .
Decisions you make as you explore eventually lead to encounters. . . .
While exploring a dungeon or other adventure location, you might try to do any of the following actions:
* Move down a hallway, follow a passage, cross a room
* Listen by a door to determine if you hear anything on the other side
* Try a door to see if it’s locked
* Break down a locked door
* Search a room for treasure
* Pull levers, push statues or furnishings around
* Pick the lock of a treasure chest
* Jury-rig a trap
The Dungeon Master decides whether or not something you try actually works. Some actions automatically succeed (you can move around without trouble, usually), some require one or more die rolls, called checks (breaking down a locked door, for example), and some simply can’t succeed.
Decisions you make as you explore eventually lead to encounters. . . .
While exploring a dungeon or other adventure location, you might try to do any of the following actions:
* Move down a hallway, follow a passage, cross a room
* Listen by a door to determine if you hear anything on the other side
* Try a door to see if it’s locked
* Break down a locked door
* Search a room for treasure
* Pull levers, push statues or furnishings around
* Pick the lock of a treasure chest
* Jury-rig a trap
The Dungeon Master decides whether or not something you try actually works. Some actions automatically succeed (you can move around without trouble, usually), some require one or more die rolls, called checks (breaking down a locked door, for example), and some simply can’t succeed.
If purifying the polluted pond is simply an instance of exploration and not a focus of the game, that is the context in which a single ritual, or (say) a single Nature check, might be sufficient. As I posted, I am assuming that we think the pond is more interesting than that and hence will be made a bigger focus of play - otherwise we wouldn't be worried about the ability of the black dragon to do it!
I believe that 4e is the only edition of D&D to have assumed that magical effects generated by NPCs and monsters outside the context of a combat encounter will be established purely via GM narration, rather than by reference to spells, spell-like abilities, special qualities and the like.
I don't know where "meaningful decisions on the players' part" has suddenly sprung from, like a rabbit from a hat; but I can't easily conceive of a circumstance in which the players knowing the mechanics for corrupt water would give enable them to take meaningful decisions that aren't available if they know that the corrupted pond is a certain size, and that there is a skill challenge on to try and cleanse it.
I have never run a "cleanse the corrupted pond" skill challenge, but here is an actual play report of a skill challenge to cleanse a fey apple grove of a haunting demon:
[sblock]
[/sblock]Notice that the stat-block of the glabrezu doesn't factor into the resolution of the skill challenge, because it is not a combat encounter. Notice also that no mechanics are needed to determine the glabrezu's ability to stop the apple trees from fruiting: this is simply a piece of fiction narrated by me as GM, having read (MM, p 55) that:
When a demon as wicked as a glabrezu lingers too long in the world, its corruption spreads. Crops are stunted, animals die, people sicken, and the sky itself seems to darken. Only when the demon is driven off or slain is the natural order restored.
I didn't need a stat block to tell me how to narrate a barren apple grove. I just narrated it. And when the PCs went to undo it, I didn't need a stat block for that either; I used the DC-by-level chart in accordance with the standard skill challenge procedures.
