Herobraine

Once the ritual is done, Herobraine watches, follows and interacts with the surrounding world. The entity switches between six behavior states depending on how close a player is, whether the player is in line of sight, and what actions the player takes. It can build small mysterious structures from stone crosses, cobblestone circles and soul torches, light netherrack fires, and switch off nearby redstone lamps.

The mod aims at psychological horror: short Darkness and Blindness effects, eerie ambient sounds, and unsettling messages sent straight into the in-game chat. The mob is difficult to defeat: it has 2000 hit points, cannot be pushed back by attacks, and takes no harm from fire, lava, drowning or falls. When attacked, it teleports behind the player before vanishing. The mod keeps only one instance within a 128-block radius to avoid duplicates and lag, and it tracks individual players, which suits multiplayer servers.

Herobraine is published for NeoForge only, for 26.1.2, and runs on client and server, so the server and every player need it. It is released under the MIT License, but it is not open source.