You have recently benefited from Kill or be Killed and cannot benefit from it again.
When you would sustain fatal damage, you instead fly into an unstoppable rage, becoming unkillable and immune to movement-impairing effects for 8 sec as you seek revenge on your would-be killer.
If you kill your killer in that time, you emerge from your blind rage triumphant, with at least 20% of your max health intact. If you are not victorious, you die.
This effect can only occur once every 5 min and if your killer is alive when you sustain fatal damage.