This is part of a series! Go here to see the other entries.
If bone and ice devils are management, then imps are door-to-door salesmen. Evil and evil-curious spellcasters love the little jerks. They’re always willing to bargain for infernal secrets of power, and look too weak to pull a fast one. Of course, that’s just how the imps and their bosses want it.
Left to their own devices, imps love tricking mortals into hurting one another, which is made easier by the fact that they’re pixie-sized, can fly and turn invisible at will. They’ll remain on any given fight only as long as they have the upper hand, and will run away as soon as the tables turn.
Imps exist both on the Monster Vault and on the Monster Manual, but the two versions are more or less equivalent. They’re Tiny Immortal Humanoids (devils), which means they’re around 6-12 inches tall - pixie-sized, as I said.
Both imp versions are Level 3 Lurkers with 40 HP. They have darkvision, Resist 15 fire, run at speed 4, and fly at speed 6 with hover capability. They’re trained in Perception, Arcana, Bluff and Stealth. The main differences are on the imp’s attacks, which we’ll discuss below.
Imp (Monster Manual)
The MM Imp’s basic attack is a bite that does so little damage it’s noticeable even on a level 3 monster. It also has a tail stinger that’s slightly more accurate, does a bit more damage, and has a secondary attack against Fortitude that on a hit does ongoing 5 poison damage and imposes a -2 penalty to Will (save ends). Both are Reach 0, which means the imp has to be occupying the same square as its target to use them. It can do that because it’s Tiny.
As mentioned above, the imp can also Vanish as a standard action, which turns it invisible for a turn or until it attacks. Using this also recharges the tail stinger, which firmly establishes the imp’s Lurker routine: vanish, sting, repeat.
The sample encounter is level 3: an imp and a whole lotta goblins, including a gobling hexer. The hexer is exactly the sort of caster who’d summon himself an imp familiar.
Imp (Monster Vault)
The MV imp has the same attacks as the original, but they’ve all been significantly improved.
Both the bite and the stinger have Reach 1 now, and do damage appropriate for a level 3 monster. The venom no longer requires a secondary attack, and does 10 ongoing poison damage while keeping its Will penalty. That’s a lot for a level 3 regular!
Vanish still works like it did before, but its text has been made more “technical”, saying that the invisibility is dispelled when the imp “hits or misses” with an attack. This is actually important, because it means the monster can make that attack while invisible and will only appear after it resolves the whole thing. You can’t see it coming beforehand, or hope to interrupt its attack by readying an action for when it becomes visible.
Imps are kind of living Team Evil membership cards. If you want a character to come off as an obvious villain, give them an imp familiar or servant. They’re also quite a bit smaller than I thought at first.
Mechanically, you should definitely go with the Monster Vault version. It retains the same flavor but is much better executed. While imp combat stats lack the pile of spell-like abilities they had in previous editions, it makes perfect sense for imps to have access to a variety of infernally-flavored rituals, and possibly for them to be able to cast them at a higher level than 3.
If you miss fireball-chucking imps, you could make artillery and/or elite versions that have similar combat powers.