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.