The mod's title entity, reacting to "show yourself."
Nothing Is Alive is one half of a player named Nothing. The Skinwalker killed him in the mine beneath his own house, and after he died in-game, it crawled out of his monitor and tore him in two. This half is the one that stayed aware, and still speaks as though it were alive, which is where the mod takes its title from.
It reacts specifically to the chat message "show yourself." Once triggered, it teleports around the player for 100 ticks (5 seconds) with unsettling on-screen text, then deals 4 damage and kicks the player from the world with "..." Nothing_is_alive answers a few things in the chat.