Our problem lies in the fact that the scenario as described is currently
ambiguous. To recap:
"So, I've got this Ashiok, Nightmare
Weaver<
http://apps.magicjudges.org/api/autocard/?card=Ashiok%2C+Nightmare+Weaver>,
and I put Desecration
Demon<
http://apps.magicjudges.org/api/autocard/?card=Desecration+Demon>
into
play with it last turn. Ajvar just played a Banisher
Priest<
http://apps.magicjudges.org/api/autocard/?card=Banisher+Priest>,
and he targeted Desecration Demon with the ability. If I kill the Priest,
does the Demon come back under my control or Ajvar's?“
Prior to the actual question, it sounds like Nutella called us over while
the trigger is on the stack. However, by asking if the Demon ”comes back“,
it now sounds like the trigger has resolved. It sounds like our first job
should be to resolve this ambiguity by asking, ”Has the Desecration Demon
been exiled?“
If Nutella answers yes, we can safely move to the answer: ”It will return
under its owner's control, in this case, Ajvar.“
Nutella might answer, ”No, I want to kill Banisher Priest in response.“ At
this point, the ambiguity is resolved the other way and we can answer, ”If
you kill the Banisher Priest in response, the Desecration Demon is not
exiled and will remain where it is.“
But if Nutella offers a simple no, we can ask, ”Are you doing anything
before the exile?" This should be enough to find out exactly when Nutella
intends to kill the Priest, and we can answer accordingly.
Without first knowing where we are and what is happening, we run the risk
of giving the wrong answer, or trying to cover all bases and coaching
players into the best course of action, even if they weren't aware of it.
Once we clarify these questions, the question that is meant can be answered.