The Diamond Throne is the setting of Monte Cook's Arcana Evolved RPG.
From the publisher (Malhavoc Press):
The release of Monte Cook's Arcana Unearthed challenged players everywhere with new options and choices. This sourcebook supports those options with more rules and provides DMs with supplemental material of their own.
The mysterious alabasts, marooned on this world for centuries, hail from a dimension unimaginably distant and command powers few understand....
Slassan are horrible amalgams of spider and serpent haunting the shadows-the legacy of a now-dead race that personified evil....