A cave-dwelling stalker with a three-phase attack sequence.
Someone Is Watching starts spawning from the third night onward, in caves below Y50 with no sky visible, on a rare roll. It now breaks itself out if it spawns inside a block.
It runs through three phases: idle for 3 seconds, then a 3-second "scary" phase where it turns to face the player, then an attack phase that chases and deals 10 damage every 0.5 seconds once within 1.7 blocks, passing through walls while doing so. If it never catches the player, it despawns after 30 seconds.