I think most of us know that if one disables demons in advanced worldgen, goblins will spawn in the underworld instead of demons. But what if, despite there being demons, there are also goblins in the Underworld? I mean, it is implied that goblins themselves were originally inhabitants of the Underworld and were only released because their demonic masters built that slade spire to let them out.
When the Underworld is revealed, instead of being in a group alongside other demons, a single(or double or trio) of unique(humanoid) demons might enter the map with a horde of 10-20 goblin “slaves”(in a rudimentary version of the master-servant relationships between demonic masters and their goblin subjects in goblin civs), with the slaves either being naked or wearing leather/bone armour and weapons/shields, crafted from the remains of other demons(presumable slain by the group). Large hordes of them(100-200) might come with Demonic Assaults(a feature I suggested a long time ago). Particularly special goblins might even receive custom slade weaponry from their “masters”, as well as have attained skills in their combat style.
These goblin-demon groups are considered Goblin Groups in Legends and battles with them are considered actual battles between groups. The only role will be “master” which is occupied by the demon “master”(who does not have a name until it starts a civilization). If retiring a fortress with the Underworld revealed, these groups may wage battles against your fortress in retirement, as well as will be given a claim in the fortress site. If your fortress is abandoned during a Demonic Assault with goblins, is abandoned when one or more of these groups are onscreen and alive or is taken over by one of these groups in retirement, the demon “master” is given a name, the group becomes a civilization and your fortress will become the capital of said civilization.
Goblins in the Underworld are immune to everything demons can throw at them(syndromes, fire, webs etc etc.), though they lose this power if they become citizens of a new goblin civilization. All goblins in the Underworld are non-hostile to demons in the underworld and are labeled as Underworld(like demons).