The concept of being "taken out" in any FATE game is not a strict matter of life-death or victory-defeat. It means one specific thing:
The player loses control of the character. The character's fate is in the hands of the opponent. The opponent is expected to narrate how the character meets his demise, in a manner that makes sense according to what brought him down. The opponent must also declare how and if the taken-out character can recover.
So in your case, whoever took out that undead thing decides what happens to it.
YS 203 says:
Getting Taken Out
If the damage exceeds the character’s stress track, or occupied boxes “push” the stress off the right side of the stress track, the character is taken
out, meaning the character has decisively lost the conflict. His fate is in the hands of the opponent, who may decide how the character loses. The outcome must remain within the realm of reason — very few people truly die from shame,
so having someone die as a result of a duel of wits is unlikely, but having him embarrass himself and flee in disgrace is not unreasonable.