Hi all, I was lending my deck tonight and the guy I lent it to got Rishkar's Expertise off Possibility Storm. I told him he had to resolve the cast from the Expertise before shuffling back the cards exiled. My reasoning was that the casting trigger would go on the stack before the shuffle to bottom trigger could resolve. Was I right in this reasoning?
If a player casts a spell due to Possibility Storm's ability, that spell doesn't resolve immediately, but rather that ability's instructions continue to be followed, which means that that player "puts all cards exiled with Possibility Storm on the bottom of their library in a random order" (which is not the same as "shuffling" as far as the game is concerned, however) (C.R. 608.2c); then the ability leaves the stack and the active player gets priority (C.R. 608.2k, 117.3b).
In general, a spell can resolve only if all players pass in a row while that spell is on top of the stack (C.R. 117.4).
Hi all, I was lending my deck tonight and the guy I lent it to got Rishkar's Expertise off Possibility Storm. I told him he had to resolve the cast from the Expertise before shuffling back the cards exiled. My reasoning was that the casting trigger would go on the stack before the shuffle to bottom trigger could resolve. Was I right in this reasoning?
Unfortunately no, you have some misconceptions here. Possibility Storm only has a single triggered ability, and once it starts to resolve, it needs to fully resolve before anything else can resolve. Your friend casts a spell during the resolution of the ability, putting that spell on the stack, but the spell can't resolve during the resolution of the ability. Possibility Storm's ability first needs to finish resolving, with the cards being put on the bottom. Then we're left with Rishkar's Expertise on the stack, then if nothing else happens in response to it, it resolves.
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I'm a former judge (lapsed), who keeps up to date on rules and policy. Keep in mind that judges' answers aren't necessarily more valid than those of people who aren't judges; what matters is we can quote the rules to back up our answers. When in doubt, ask for such quotes.
In general, a spell can resolve only if all players pass in a row while that spell is on top of the stack (C.R. 117.4).
See also this thread.