I have been going through old threads on the old Wizards forum. terminus_vortexa mentions the links between the Outer Planes of the standard D&D cosmology and the non-standard Athasian cosmology.
Dregoth made a deal with a baatezu warlord. The undead dragon king needs troops to wage his war against the city-states of the other sorcerer-kings. The baatezu seek to dominate the Prime Material Plane and its natives to gain an advantage in their long-fought Blood War with the tanarri. Dregoth, of course, believes that the baatezu will obey him. In truth, they only need Dregoth to show them the way to Athas. Then they plan to take control of the world and its inhabitants-including the Dread King. This small band of green abishai were the first baatezu to step through the planar gate. The others wait patiently for Dregoth to open the portal again.
The Planar Gate
You gaze upon a large, ornate mirror. The glass does not reflect your own image, however, but a scene from some other world. Instead of sand and heat, everything through the mirror is covered in solid whiteness and it looks very, very cold. A vast army of terrible creaturessome like the ones you fought here and in Kragmorta, others like red, scaly giants with wingsreadies itself for war.
The planar gate currently looks upon Baator and the icy realm of Stygia. An army of baatezu patiently wait for Dregoth to open the Baator side of the gate so they can march on Athas.