Undercover Goblin

When Undercover Goblin enters the battlefield,
target opponent gains control of it.
Undercover Goblin attacks each turn if able.
When Undercover Goblin attacks, exile the top
card of your and its owner's library. You and
its owner may play the cards you exiled until the
end of their next turn, and they may spend mana
as though it were mana of any color to cast spells exiled this way.
"He knows we can tell, right?"