There was a discussion on a discord channel about the "Unknown Mosnter" that has an AC -3. Someone tagged me and said I should whip up stats for it quickly as a joke, but I went ahead and did it. It was done in about 5 minutes, don't judge me too harshly.
A monster so horrible that it cannot be written about in books, and to view it is to go insane.
Armor Class -3 
Hit Dice 26+*** (117 hp)
Attacks 2 × things 4d10
THAC0 5 [+14]
Movement Rate 120’ (40’) Flying 120’ (40’) Swimming 120’ (40’)
Saves D2 W2 P2 B2 S2 (M36)
XP for Defeating 8500
Number Appearing 1
Treasure Type Nil
- Damage Immunity: Can only be harmed by +3 or better magical weapons or magical spells.
- Regeneration: Unknown Monster regain 10 hit points a round.
- Confusion: Looking directly at the Unknown Monster causes a save vs spell or be confused for 1d6 rounds.
- Cast Death Spell
- Cast Teleport
- Destroy one non-magical item it can see.
- Create a large unknowable cloud