Another of those big beaters with a downside from back in the day for the early Johnnies
Getting back a Dark Ritual and a Cabal Ritual when you have threshold, for example, is effectively to pay four mana for cards that net you five mana (Dark Ritual is +2, Cabal Ritual w/ threshold is +3)
It doesn't matter if the blue and/or black spell an opponent cast that turn resolved, didn't resolve, was countered, or is still on the stack
Its whatever you need at that particular time
It has already decided and is just keeping us in aim
Of course, in hydra tribal, even better, but also if you have a deck with clones, clones of the back of Polukranos here will see each other die to trigger multiple times for a geometric amount of hydras