The River Warlock [Unknown Event]

During your turn, The River Warlock has flying, lifelink, and indestructible.
Whenever The River Warlock attacks, you and defending player each draw a card, then discard a card. If you discarded a card with equal or greater mana value than the card that player discarded, look at their hand. Exile a nonland card from it until The River Warlock leaves the battlefield.