Pyrexis, Living Doomsday

This creature costs {x} less to cast, where X is the total amount of damage opponents have taken this turn.
Whenever a source deals damage, put a Doomsday counter on Pyrexis, Living Doomsday.
{4}{r}{r},{t}: Sacrifice this creature, then deal X damage to each opponent, where X is the number of Doomsday counters on it.