This mod adds the Invalid Entity from Bionic's movie, Beyond The Border. Version and mod loader: 1.20.1 Forge. Watch the movie if you don't know or saw the entity. Mod ID: invalid.

Here is how it looks like:

It has white eyes with a black body.

Here is the texture file:

The reason on why it's completely filled in, is because to remove the holes in version 1.2.0. Only the eyes are showing.

This mod also adds the following blocks, items and biomes:

Invalid Spawn Egg (entity name: entity.minecraft.invalid)
Invalid Block
Invalid Biome (id: invalid_biome)
Invalid Fluid (in-game name: minecraft.item.invalid_fluid_bucket)

This mod also adds Steve to Minecraft as a passive mob, meaning he will never attack you.

To summon this entity, run /summon invalid:invalid. For Steve, type /summon invalid:player.

This entity will try to kill you if you hit it. You can use a morph mod like Identity and Remorphed to morph into this entity and even troll your friends.

This entity is found when you break the World boundary using an NBT editor and going to X/Z ±30000900 and making sure the game still thinks X/Z ±30000000 as an invalid position.