Bolas's Assassin

Bolas's Assassin is unblockable as long as it is attacking a planeswalker.
Unearth {u}{r}
Whenever Bolas's Assassin deals damage to a planeswalker, put a +1/+1 counter on it.
"From the rooftops of Ravnica, to the citadels of Bant,
to the jungles of Tazeem, I have killed many for the
dragon."