Sif, Great Grey Wolf

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

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

[Sarulf, Realm Eater]