Loki, Master of Trickery

Loki starts the game in exile. At the beginning of
each end step, return Loki to exile
Whenever you counter a spell, you may pay {2}{u}{b}. If you do, put Loki from exile onto the battlefield.
When Loki enters the battlefield, you draw 2
cards and target opponent discards 2 cards.
Choose 2 creatures that opponent controls.
Those creatures can't attack this turn.