Originally posted by magnuslives
i missed the whole legion thing - didn't he lead to AoA or something by killing his dad by accident? in that case he caused a LOT of havok to say the least.
Yup, he traveled back in time and killed Xavier on accident. He used some sort of psychic knife, one that would put Betsy's to shame.
Legion led to the destruction of the original 616 timeline. The current one is different and has been slightly altered, since Bishop traveled back in time and stopped Legion. So technically Legion killed every being in the 616 universe.
Cassandra Nova is responsible for the death of 16 million mutants and near fall of the shi'ar empire.
Onslaught killed a couple of heroes that were resurrected a while later. That doesn't compare with anything CN and Legion did.