The Arcane are a race from the multiverse prior to this one, and partially responsible that destroyed it. They live today only as visitors, attempting through their machinations to recreate their lost reality, a paradigm where art took precedence over natural law.

Alternatively, they're minions of a lost god who abandoned them (Velnimach, if you have the Primal Order book). they collect magic items in the hope of creating a god of their own who can create more of their kind. To this end they have quarantined many entire worlds, filling them with the spoils of their trades to feed the embryonic deities they keep there.

Arcane exist partially on another reality loop and show it, having little interest for the affairs of the world they're marooned on. They deal with mortals through shaped intermediaries and don't hesitate to "skip" the surface of existence if their present company wearies them (dimension door). Only a few Arcane will ever be met by the PCs, perhaps with distinctive characteristics (artificial hand, eyepatch) or perhaps corresponding to the arcana of the Tarot. They're aware of each others' activities at all times.

A race particular to the sudden jungles in as unlike the Arcane as conceivable: gregarious where the Arcane are aloof, and full of shimmering colors and constantly shifting shape. The "truth" is that they are part of the same race.

Mechalich's ethergaunt article