If any character in your party gets on death saves then your character gains an extra d12 to all attacks and cant miss but you don't know your hp your dm does and you are unable to heal, take bonus actions and reactions. Also you cant hear anything around you and all buff that are on you and all buffs to be put on you fails. This feat end when ever your character dies or the fight is over. Also if you fight that same enemy again this feat will trigger