From the publisher's website:
Cthulhu Dice lets you drive your rivals mad . . . very, very quickly. Players take turns rolling the big, beautiful, custom 12-sided die, embossed with tentacles, Elder Signs, and more. Destroy your opponents' sanity! Better yet, steal it. But watch out for Cthulhu – when he comes up, he takes sanity from everyone! 18 glass Sanity marbles are included. Lose all your marbles and you're mad. The last sane cultist wins . . . unless everyone goes mad together. Then Cthulhu wins!