Riverbane Pteron

Flying, Flash
When Riverbane Pteron enters the battlefield,
you may mill up to three cards, then put a +1/+1
counter on Riverbane Pteron for each creature
card that was milled this way.
Whenever Riverbane Pteron attacks, you may
pay {2}{u/b}. If you do, return target creature to its
owner's hand at the end of turn.