Potion of elemental form
From Ardrana
There are four types of the potion of elemental form: Earth, Air, Fire, and Water. The user may change into the form of an elemental (of the appropriate type) and back to normal form as often as desired while the potion lasts. Each change of form takes about a minute. While in elemental form, no special immunities are gained, but the special attacks of each elemental are usable.
Note: This item originally appeared in this form in the D&D Rules Cyclopedia for the D&D game from TSR. Their use here is for the purposes of providing context for the campaign only.