Upon dying, Kog'Maw becomes ghosted gaining 10-50% bonus movement speed over 4 seconds and starts a chain reaction in his body which causes him to move faster and detonate after 4 seconds, dealing 140−650 true damage to surrounding enemies.
This passive makes, as stated above, you able to revenge your death. The difference between this passive and the one karthus has is that karthus won't be able to move, so people will be able to get away.
Khuh!
Way to go Kog'Maw!!
Bet you can't freaking do it again!!!
--Summoner(Walter)