Knowledge Pool

If the card you cast without paying its mana cost has an X in its mana cost, you must choose 0 as its value.

If multiple Knowledge Pools are on the battlefield, keep track of which cards are exiled by each of them. Whenever a player casts a spell from their hand: -- If all Knowledge Pools are controlled by the same player, that player chooses the order in which the triggered abilities are put onto the stack. The last one put onto the stack will be the first to resolve. -- If multiple players each control one or more Knowledge Pools, the active player put their triggered abilities on the stack in any order, then each other player in turn order does the same. The last ability put onto the stack this way will be the first to resolve. -- The first triggered ability to resolve will exile the original spell, then the player who cast that spell may cast one of the nonland cards exiled by the Knowledge Pool that generated that triggered ability. The abilities of other Knowledge Pools will do nothing when they resolve, as the original spell will already have been exiled.

If the card has any mandatory additional costs, as Kuldotha Rebirth does, you must pay them in order to cast the spell.