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The Magma Mephit has a neat little ability called Change Shape:

Change Shape (Su) Once per hour, a magma mephit can assume the form of a pool of lava 3 feet in diameter and 6 inches deep. The mephit’s damage reduction improves to 20/magic when in this form. The mephit can’t attack while in lava form but can use other spell-like abilities. It can move at a speed of 10 feet, but it can’t run. In this form the mephit can pass through small holes or narrow openings, even mere cracks. The pool’s touch ignites flammable materials such as paper, straw, or dry wood.

Since Su abilities default to a standard action, we can forgive that the ability has no listed action cost.

But how on Oerth does it turn back into a mephit? Does it do this at-will, or does its magma form expire somehow?

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up vote 12 down vote accepted

The rules as written for the special ability change shape provide no mechanism for reverting to the original creature once the creature's used the special ability unless explicit in the creature's change shape ability. This means, for example, a barghest having once changed into wolf shape or goblin shape is forevermore incapable of assuming barghest shape, much like the magma mephit upon changing shape into a pool of lava is stuck forevermore as a pool of lava despite it being able to "change shape" once per hour ("Ta da! I'm a pool of lava. [An hour later.] Ta da! Hey, I'm still a pool of lava. [An hour later.] Ta da? Still a pool of lava? Argh!")

By the Monster Manual III, it appears developers noticed this dysfunction. For instance, the rakshasas therein have the special ability change shape but the description includes them being able to revert to their original forms as a standard action. A DM wanting actual barghests in his game instead of creatures that can switch between goblin and wolf shapes and say they're barghests or a DM wanting magma mephits who can change shape from a pool of lava and back into a magma mephit should probably extend creatures that can't revert to their original shapes the same courtesy as the Monster Manual III's developers did to their creatures and let them revert as a standard action.

(Although the Monster Manual IV has a glossary entry for change shape, no creatures within have the ability. Monster Manual V omits even the glossary entry.)

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I'm not certain what book you are using, but the Monster Manual states:

"Spell-Like Abilities: Once per hour, a magma mephit can use shapechange to take the form of a pool of lava..."

The duration is thus equal to 10 mins/caster level (D); The action cost is the same- one standard action; as a spell-like ability, the effects expire when the duration has reached its limit or when the user chooses to end the effect as a standard action, as normal.

The only thing that isn't specifically stated in the monster's description is the all-important caster level of the spell. After doing a quick survey of the other mephits, I think it is safe to assume that the caster level for shapechange is equal to 3, as every other mephit has one spell at CL 3 and one spell at CL 6, and the magma mephit's pyrotechnics is specifically stated as being CL 6.

Sources

Monster Manual 3.5; Player's Handbook

TL;DR

Shapechange as the spell; CL 3 = 30 minute duration

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3  
Page 4, column 2 of the Monster Manual I errata rewrites the ability; the d20SRD link above matches the errata text. – SPavel Feb 3 at 22:10
    
Thanks, somehow I missed that errata. Should double check them just in case. – Dharenis Feb 3 at 22:43
    
Down vote because it's incorrect (as SPavel points out), but good answer otherwise. – fectin Feb 4 at 14:06

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