#ability duration

[magicjudge]

This happened to me and enough people disagreed with me I let it happen, since I didn't understand the ability well enough to know, myself. If a opponent casts Act of Treason on my Kotose, the Silent Spider, can they then cast the card I previously exiled with her triggered ability?

Good news: Your opponent cannot cast an exiled card. The entire effect is part of her ETB triggered ability, so changing control of Kotose doesn't suddenly allow someone else the ability to cast it. (It would need to be a separate ability on Kotose for that.)

Bad news: You can't cast the card either anymore, since you've now lost control of Kotose and even when you get her back in the cleanup step, the duration for her last clause has lapsed and can't be restored for any card she has already exiled.

[magicjudge]

I took a call at work last week about how Fairgrounds Warden was different from Fiend Hunter (FH is two separate triggers, FW is one, roughly), but the follow-up stumped me: why is the creature FW targets never exiled at all if FW leaves the battlefield before his trigger resolves? I figure it's something to do with the ability naming an object that's no longer in play, but I wasn't able to articulate why this happens the way it does.

If an ability has a limited duration and that duration ends before it would begin, the effect never happens at all.

[magicjudge]

What happens to the aegis angel ability when entering the battlefield when someone uses vapor snare?

When Vapor Snare’s controller gains control of the Aegis Angel, the duration of the Angel’s ETB effect ends and the targeted permanent is no longer indestructible.