He's Watching

The mod adds an entity that spawns rarely, only on solid ground with a clear line of sight and enough light. It appears in the peripheral vision band, never straight ahead and never dead center, and it leaves once the player looks directly at it. If the player turns away and back, the figure is usually gone, and it sometimes returns a few minutes later somewhere nearby.

Several settings control how the entity behaves: how often it appears, the cooldown between appearances, how close or far it spawns, how wide the peripheral band is, how precisely the player must look to make it disappear, how long it waits before giving up, and the minimum light level needed for a spawn. A chance to delay a rare second appearance and a rule that favors darker, less-watched sides are also part of its behavior.

Installation is needed on both sides, since the environment is client and server. Files are published for Forge and NeoForge for 1.20.1 and 1.21.1, with 1.19.2 also in the range of game versions that have files, and the mod is released under All rights reserved, so it is not open source.