The Unknown Force is a crystallized man.
The Unknown Force's files were found to be a cactus as a placeholder for a secret update, maybe?
- Minecraft 1.7.2: Unknown Force first seen.
- Minecraft 1.7.2: Files investigated and cactus found.
- Minecraft 1.7.2: The cactus image updates to several other blocks, and finally ends at an unknown item defined as a crystal.
- Minecraft 1.9: Crystal added.
- Minecraft 2.1: Unknown Force added to the game as a crystallized man.