From Portals of Phereon Wiki

A genetic trait that gives a boost to Magic and Mana. Keep an eye out for this trait in battles, as a unit with Possessed will turn into a Spectre when killed.

This means enemies with this trait will need to be killed twice over, but the Mana drain effect can also be used to your advantage by essentially crippling the enemy team's spellcasters.

This trait is also the only way to acquire Spectres (which don't spawn naturally) and the Ectoplasm they produce, which is needed for some very useful recipes.