Not a pantheon. Not a nation. A cabal old enough that most of their names outlasted the reasons anyone first feared them.
The Cadaverous Coven is a terrifying cabal of five ancient, pre-eminent undead monstrosities, bound by a singular ambition of claiming the Divine Well to ascend as the new pantheon of the mortal realm. Long ago, four of the five were sealed away across the world within monumental prisons, secured by five elemental relic keys. Only King Horkos escaped confinement, having located the Divine Well and achieved divinity before he could be sealed away. Now, clandestine cults, death-worshiping cabals, and any number of vile, evil or insane, scour the continents to recover the keys and unseal those remaining.
A mortal warrior king of Seug who forged a dark pact rather than let his natural life run out, became a Death Knight, and eventually found the Divine Well and ascended to full godhood. The newest name in the pantheon, and a confirmed Coven member, the only one to have ascended so far.
An aberrant creature called the Nyarlathen, a monstrous horror from some distant plane of existence, of awesome power in the arcane and psionics, that underwent the ritual sealing away its essence and becoming a lich. Currently locked away, sealed behind the Electric Key.
A ravenous, gluttonous undead spirit. No true name is recorded for it, "The Hollow Maw" is the name history settled on. Currently locked away, sealed behind the Stone Key.
The world's first vampire, created by William Silverrose during his early adventuring days and the creator of every vampire bloodline that followed. Bloodthirst eventually corrupted him; he's now imprisoned in an Iüdex temple beneath the Ice Shelf, along a subterranean river of lava, sealed behind the Air Key.
The unconquered martial emperor of pre-glacial Korone, embalmed in liquid gold and cursed linens before the ice took his homeland. Sealed inside a step temple in the jungles of Pyth behind the Adamantine Ice Key Mold.
The un-ascended members of the Coven are each sealed away by a specific elemental key. Horkos needed none, he found the Divine Well before anyone thought to build him a cage. Finding, or protecting, the remaining four keys is a central campaign hook.
Seal of Golaric Noxar. An almost two-foot-tall platinum scepter topped with a massive, nearly clear diamond, its shaft covered in intricate dead-language glyphs.
Seal of the Ice Lich. An octagonal reliquary box of electrum, storm-glass, and blackened silver, hinged into eight facets around a hidden eight-pointed star shaft.
Seal of the Hollow Maw. A monolithic granite key roughly three feet long, far too heavy for normal carry, though a hidden command word compresses it to pocket size.
Seal of the Nameless Dread, Warlord of Korone. A pair of adamantine molds that must be filled with water and frozen into a key, then rushed to the lock before it melts in the jungle heat.
Reference art for the Coven's imprisoned members.