Say I have Sengir Autocrat with his 3 serfs out. Am I able to sacrifice him, his 3 serfs, and an additonal creature to pay for Westvale Abbey's transform ability? Or does his effect make it impossible to sacrifice him together with his tokens, as him leaving exiles all tokens leaving me with only 2 creatures (Sengir Autocrat and the additional creature) to sacrifice?
Yes, you may sacrifice all three serf creature tokens to help pay Westvale Abbey's Costs.
This is because Sengir Autocrat's second ability is triggered when he leaves the battlefield. By the time you've finished activating this ability, all your serfs have already been sacrificed to the Abby and are no longer on the battlefield to be removed.
Let's break down the interaction step by step:
- You have a Westvale Abbey, Sengir Autocrat, 3 serf tokens, and a zombie token.
You activate Westvale Abbey's ability, tapping it, paying five mana and sacrificing all 5 creatures. All the creatures goto your graveyard per comprehensive rules:
701.15a To sacrifice a permanent, its controller moves it from the battlefield directly to its owner’s graveyard. A player can’t sacrifice something that isn’t a permanent, or something that’s a permanent he or she doesn’t control. Sacrificing a permanent doesn’t destroy it, so regeneration or other effects that replace destruction can’t affect this action.
With its costs paid Westvale Abbey's ability goes onto the stack.
- The game sees that Sengir Autocrat has left the battlefield, his ability goes onto the stack.
- Assuming no responses, the stack starts to resolve. Sengir Autocrat's ability resolves and checks for any Serfs you control, but they are already gone so nothing happens. Westvale Abbey's ability now resolves and you get your Profane Prince.
You can sacrifice the Serfs and the Autocrat together.
This is a triggered ability:
When Sengir Autocrat leaves the battlefield, exile all Serf tokens.
The moment Sengir Autocrat dies, all that'll happen is that the ability goes on the stack. You can respond to it normally, including by activating other abilities, before it resolves and you're forced to exile the Serfs. (You could even use Time Stop or Stifle to counter the trigger and keep the masterless Serfs "forever.")
"Sacrifice five creatures" is a single game action. You do the whole thing, then check for state-based actions. The new game state (Sengir Autocrat went to your graveyard) will cause its ability to trigger. Then and only then will you put its ability ("exile all Serf tokens") on the stack.
In fact, even with a card like Master of Waves, which kills its token-children as a state-based action when it leaves play (and they end up with zero toughness), you can sacrifice the lord and all the tokens together. Because even state-based actions aren't checked until you perform the full action — "sacrifice five creatures" (also, tap the Abbey and pay five mana). It's one discrete "moment" as far as the game is concerned, even if you spend a full minute picking which creatures to sacrifice as you're activating the ability in real life.