Sneaky Goblin

Instead of paying this cards mana cost, at any
time you can put Sneaky Goblin under a
creature card you control. If you're opponent
catches you, sacrifice Sneaky Goblin and lose
5 life.
If a creature with Sneaky Goblin under it deals
combat damage to an opponent, reveal Sneaky
Goblin from underneath the creature, and that
opponent loses the game.