Koh, the Face Stealer

Whenever another creature enters the battlefield,
you may pay {x}, where X is the creature's
converted mana cost. If you do, Koh, the Face
Stealer becomes a copy of that card, except its
name is Koh, the Face Stealer, it's legendary in addition to its other types, and it has this ability.
The creature you copied can't attack or block,
and it loses all abilities until Koh, the Face
Stealer copies another creature.