I was playing my friend tonight and a crucial play depended on me using Capsize on his Altar of Dementia before he could sacrifice a creature to it, triggering that creature's death ability. He did not accept my play.
My question is, even though Capsize is an instant, can he still activate Altar of Dementia? It was on my turn if that matters at all. Would it work if I had played Capsize in response to his sacrificing? Is there any way to get around this?
Thank you.
You can't use Capsize to prevent your opponent from sacrificing the creature.
If you cast Capsize, your opponent can sacrifice the creature with Altar of Dementia's ability in response (C.R. 116.7), and since doing so is a cost (C.R. 602.1), that creature will leave the battlefield before Capsize can get to resolve (C.R. 601.2h-i, 116.3d, 116.4).
If you wait until after your opponent activates Altar of Dementia's ability, Capsize can no longer target the sacrificed creature anymore (C.R. 601.2c) because, again, the sacrifice is a cost and that creature will leave the battlefield before you get priority to cast Capsize (C.R. 601.2h-i, 116.3d).
Whose turn it is doesn't matter here.
In general, the only way to keep your opponent from sacrificing that creature that way is to use some means to prevent your opponent from activating Altar of Dementia's ability.
Thank you very much for the response.
So a Stifle or a Disallow would work?
Neither spell would keep the creature from being sacrificed with Altar of Dementia's ability. Although both spells can counter abilities, keeping them from resolving (C.R. 701.5a), they don't "refund" any costs already paid for them (for example, sacrificed creatures don't return to the battlefield) (C.R. 701.5b).
EDIT: Clarified my answer after comment 6 was posted, and again after comment 7 was posted.
You could do that. Again, countering an ability keeps it from resolving (C.R. 701.5a), so it won't have an effect (which means that in the case of Hallowed Spiritkeeper, no tokens will be created).
I was playing my friend tonight and a crucial play depended on me using Capsize on his Altar of Dementia before he could sacrifice a creature to it, triggering that creature's death ability. He did not accept my play.
My question is, even though Capsize is an instant, can he still activate Altar of Dementia? It was on my turn if that matters at all. Would it work if I had played Capsize in response to his sacrificing? Is there any way to get around this?
Thank you.
If you cast Capsize, your opponent can sacrifice the creature with Altar of Dementia's ability in response (C.R. 116.7), and since doing so is a cost (C.R. 602.1), that creature will leave the battlefield before Capsize can get to resolve (C.R. 601.2h-i, 116.3d, 116.4).
If you wait until after your opponent activates Altar of Dementia's ability, Capsize can no longer target the sacrificed creature anymore (C.R. 601.2c) because, again, the sacrifice is a cost and that creature will leave the battlefield before you get priority to cast Capsize (C.R. 601.2h-i, 116.3d).
Whose turn it is doesn't matter here.
In general, the only way to keep your opponent from sacrificing that creature that way is to use some means to prevent your opponent from activating Altar of Dementia's ability.
EDIT: Clarification after comment 7 was posted.
So a Stifle or a Disallow would work?
EDIT: Clarified my answer after comment 6 was posted, and again after comment 7 was posted.