Viscorai, Unnatural Selector

Swarm (Whenever this creature deals damage
to a creature, put X +1/+1 counters on it, where
X is the number of mana symbols in that
creature's mana cost.)
Whenever another creature with Swarm
enters the battlefield, it gets +0/+4 until
end of turn. You may have Viscorai fight
that creature. If you do, Viscorai gets
+0/+4 until end of turn.