Fenrir, World Eater

Whenever a permanent an opponent controls is put into a graveyard from the battlefield, put a +1/+1 counter on Fenrir.

At the beginning of your upkeep, if Fenrir has one
or more +1/+1 counters on it, you may remove all of them. If you do, exile each other nonland permanent with mana value less than or equal to the number of counters removed this way.