If a player controls two or more creatures tied for the greatest power among creatures he or she controls, that player chooses one of them to sacrifice.

The sacrifice is not dependent on the damage being dealt. It doesn't matter if that damage is prevented or redirected.

Crackling Doom doesn't target any player or creature. For example, a creature with protection from black could be sacrificed.

In a multiplayer game, each opponent, starting with the active player and proceeding in turn order, chooses which creature he or she will sacrifice and then all creatures are sacrificed at the same time. Each successive player will know what the players before him or her chose to sacrifice.