Xarith Sepa

Islandwalk
{2}{u}: Until end of turn, Xarith Sepa becomes a blue Puddle with base power and toughness of 0/1, indestructible, and hexproof that can't block or be blocked. If Xarith Sepa attacks and is not blocked as a blue Puddle, defending player puts the top 10 cards of his or her library into his or her graveyard, then draws a card.
"Don't slip now."