QI put this follower onto my field from an opponent's cemetery with the On Evolve ability of Octrice, Omen of Usurpation, making me its new controller.

If the Eternal Whale on my field is destroyed, what happens?

AYou and your opponent would resolve the interaction as follows:

1. The Eternal Whale is destroyed and goes into your opponent's cemetery. While you were the controller, your opponent is the card's owner.

2. Eternal Whale's Last Words trigger, and you play the ability.

3. You resolve the Last Words, dealing 2 damage to each enemy leader.

4. You choose whether to place Eternal Whale 2nd from the top of your opponent's deck, or on the bottom. Since you are not the owner of that deck, your opponent places the Eternal Whale where you designate.