Xethelil, The Lurker

Swampwalk, Evolve (Whenever a creature enters the battlefield under your control, if that creature has greater power or toughness than this creature, put a +1/+1 counter on this creature.)

When Xethelil enters the battlefield, choose a letter. Creatures you control that start with that letter are unblockable. When a creature that starts with the chosen letter enters the battlefield, you may draw a card.