Sif, The Gray Wolf

Whenever a permanent an opponent controls is
put into a graveyard from the battlefield, put a
+1/+1 counter on Sif
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 mana value less than or equal
to the number of counters removed this way.