And is this is also true for rebound ? And another question came to mind: When using knowledge pool player casts a spell, it takes effect, then got exiled or you pay it's mana cost, it at once goes to exile with no effect?
Rebound only applies to spells cast from your hand. If a spell is cast from your hand, however, Knowledge Pool 's triggered ability exiles the spell before its rebound allows it to be exiled for re-casting the next turn.
Yes, a rebounded spell will go to the graveyard as the final part of its resolution like normal for any non-permanent spell.
When you cast a spell from your hand (selecting targets, paying its costs, etc.) Knowledge Pool 's trigger will exile that spell. It's removed from the stack before it can resolve, so it cannot resolve and therefore has no effect. You get another spell "for free" from the Pool. Note, that the Pool's trigger effect doesn't happen instantly, it can be responded to.
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