Bladegriff Prototype — ruling 1

Even though the player who was dealt damage chooses the target for Bladegriff Prototype's ability, you are the controller of the ability.

Bladegriff Prototype — ruling 2

If an opponent dealt damage by Bladegriff Prototype leaves the game before the triggered ability goes on the stack (most likely because that player's life total was reduced to 0 or less in combat), you choose another opponent to choose the target permanent.

Bladegriff Prototype : règles, fonctionnement et interactions (MTG)

Bladegriff Prototype is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Flying Whenever this creature deals combat damage to a player, destroy target nonland permanent of that player's choice that one of your opponents controls.

How it works : Bladegriff Prototype

Original English text — Wizards of the Coast / Scryfall

#1November 10, 2020· Wizards of the Coast
Even though the player who was dealt damage chooses the target for Bladegriff Prototype's ability, you are the controller of the ability.
No other card is named in this ruling.
#2November 10, 2020· Wizards of the Coast
If an opponent dealt damage by Bladegriff Prototype leaves the game before the triggered ability goes on the stack (most likely because that player's life total was reduced to 0 or less in combat), you choose another opponent to choose the target permanent.
No other card is named in this ruling.