Jaila's years of using shadow magic has affected her physiology to the point where constant exposure can cause her to transform into an elemental of that power. Isis, and any member of Jaila's family lineage worth their salt as magic users can obtain this form. Unlike Dira-Beasts (something this form is many leagues below) which are forms of weaponized instinct the user still has some control over their actions and can direct their power as needed, but it isn't the most glamorous state to be in.

The power boost in general, and the ability to do greater damage against a light element aligned opponent are upsides, but of course being more sensitive to light magic to the point where a simple light bolt, if not well defended against can hurt far worse then a simple sting. The mind must be sturdy, for entering this state can entrap the user's mind, and imprison it for an untold period of time leaving the user doomed to remain in this state for an infinite amount of time, until the body's base needs overtake it, and thus expire the user in turn.

This isn't limited to just her of course as mentioned, all magic users can be affected this way.

Jaila © to  Jeffron