Screaming Nemesis

If Screaming Nemesis is dealt damage by multiple sources at once, such as by two creatures blocking it, its last ability triggers once and one target is dealt that much damage.

If lethal damage is dealt to Screaming Nemesis, its last ability still triggers.

Once Screaming Nemesis's last ability causes it to deal damage to a player, that player won't be able to gain life for the rest of the game. It doesn't matter if Screaming Nemesis remains on the battlefield or not.