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.
The adventure in CbtSS assumes the PCs destroy the gate, which means there’s no known or easy way for the Baatezu to come through to Athas. It’s possible Dregoth could create an Epic spell to let him communicate with the Baatezu again, but we come back to the transport method.
I’d guess after a short period of time and the Baatezu confirming the portal was destroyed, the Stygian warlord moves on to other things.
If they ass pass through the gate, at least initially they’re likely to go along with Dregoths plans. I mean why not? Their goals align, so there’s no gain to betraying him. They’re also Lawful, capital L. Later they may find control of the gate beyond them (for reasons), at which point they can continue Dregoths conquest. They still gain substantially from the deal, at little cost.
Until the other SKs arrange the Gates theft to thwart Dregoth, expand Nibenays knowledge and release Andropinus (or attempt it at least), employing the PCs.
I had them steal it and deliver it. Andropinus needed ot to escape the Black but couldn’t use it from his location so had to hope Nibenay would honour their agreement to help free him using it. Few others having the knowledge to even attempt it. Nibenay himself needed to study it before any attempt, so it wasn’t an instant solution anyway, which Andropinus knows but can’t do anything about. Just hope.