Mage Eater

When Mage Eater enters the battlefield, you
may tap a creature. That creature cannot be
untapped as long as Mage Eater is on the
battlefield. Add a +1/+1 counter to Mage Eater
for each mana the tapped creature cost.

Mage Eater gains +1/+1 for each spell you cast
until end of turn.

"It's a fate worse than death."
-Tristan, Wizard Adept