[Soyweiser]

Eating a mage is a very big thing. Mages win games(*). So it isn't that useful.

iirc only la ulm has it. But there might be one other nation that has it.

(*)Actually mage turns win games. A mage can only do one thing each turn, search, forge, move research etc. By using possession you use up the mage turns for the rest of the game. Usually, this isn't worth it. Even with cheap mages. (As recruiting a mage also costs a fort turn).