The beast within you is constantly coiling and uncoiling, waiting for a chance to break free. When you kill, this rouses the beast and it might take control.

Whenever you reduce a creature to 0 hit points, you must make a Wisdom saving throw with a DC equal to 10 + your proficiency bonus.

On a failure, you lose control and are compelled to use your action each turn to make a melee attack against a target you can reach— whether friend or foe. You make this attack with advantage. If you cannot reach any target, you are compelled to move as close to the closest one as quickly as possible.

You can repeat this saving throw at the end of each of your turns and snap out of it on a success, or when you are incapacitated.