D12

D12 is the loneliest die. It is used for barbarian hit dice and greataxe damage in Dungeons & Dragons 3e.

The fact that Orcs (and Half-Orcs) are both the most common barbarians and the most common wielders of greatatxes, it is suspected that Gruumsh is the head of a conspiracy aiming to eliminate the d10 in favor of the d12, in order to "purify" dice sets so that they consist only of true platonic solids.

Observant smar/tg/uys will notice that d12 are the hitdice used for undead and dragons instead of the usual d8s.

D12 is the highest stats can ordinarily go in Savage Worlds. BBEGs and DMPC Mary Sues can have D20, but it's hella rare.

D12 shows up in Cthulhu-themed games, like Pokethulhu or Cthulhu Dice, probably because d12 is the mutant freak of dice.