Cosmology

While no one knows exactly how Mirosia was born or created, it's Rifts have been operating for millennia, pulling through all sorts of artifacts, temples, and various followers of divine beings. As such, as their followers and places of worship were drawn into Mirosia, various gods, both good, evil, and otherwise became aware of it. This has had a unique result in that more than one pantheon of divine beings has access to Mirosia, ranging from deities of the Forgotten Realms to Eberron, and even some of the Nordic and Greek pantheons. These divine beings have for the most part agreed to leave each other in relative peace - with one exception.

The Dark Gods
While most of the good and neutral deities have agreed to respect one another, evil deities have no such interest in co-operation - at least not one sufficient to motivate them to work with others to any great degree of co-ordination. The evil gods have a loose alliance to help one another, preventing the other gods from ganging up and overwhelming them. However, each has their own agenda, which don't always align with one another. These deities have collectively been deemed "The Dark Gods" due to their association with evil.

Prominent members of the Dark Gods include deities such as Tiamat, Lolth, Eberron's Dark Six, as well as Vecna and Strahd.

The Weave of Mirosia has another complication that makes life difficult for the dark gods - the weave cannot sustain fiends, and they disintegrate and cease to exist upon arrival in Mirosia, regardless of their power level. Tieflings from other realms are subject to this phenomena as well - they only exist in Mirosia as a result of random mutations that may occur to those unlucky enough to fly into Limbo Fields. The effect is even extensive enough to affect the mind - the very idea of what a fiend is simply forgotten and erased from a living being's mind upon entry to Mirosia, and even written texts and other forms of record will be erased.

As such, the Dark Gods cannot manifest directly, and must rely on other forms of servant, such as undead, necromancers, chromatic dragons, dark fey, driders*, and other manner of mortals and monsters.

''*Driders are unique in Mirosia. Some arrived from other lands, cursed and mad. Others have originated in Mirosia, drow that have taken the curse on purpose as a blessing from Lolth due to her lack of fiendish servants. These driders are surprisingly sane (for religious fanatics, at least), have great skill in making elaborate plans, and often have talent as clerics or warlocks in Lolth's service.''

Primordials and Aberrations
Last of all, various primordial beings and aberrations from beyond have also found their way to Mirosia. Their allegiances and agendas remain unknown.