Dabus. They speak to each other with those illusions. They're servants of the Lady of Pain, who rules the City of Doors and keeps it safe. . .
The dabus are the servants of our Dread Lady, Her Serenity the Lady of Pain. Her will is their will. They are also Sigil’s first settlers, more native than the planars who just happen to be born here. There are no records, no tales, not even rumors of a time in Sigil when the dabus were not present, silently watching over the City of Doors.
Some of the wise say that the dabus built Sigil, and that’s why they serve it as no other Cagers do. Dabus seem to consider Sigil their master as much as the Lady, for they are forever patching and fixing it, laying cobbles, digging up pipes, trimming back razorvine, roofing city buildings, whitewashing, and sweeping the streets. Likewise, they often tear down sections and build over streets that they find unworthy (for reasons known only to themselves). Oddly, when the dabus are questioned they claim that the city itself created them. No one’s quite clear on what they mean by this, and they rarely elaborate. The few vague explanations they do offer are completely obscure.
The homes of the dabus are deep underground; some Cagers say that the entire torus is a warren of dabus, and the part of Sigil on its surface is only the face the city shows to the Ring, to travelers. The actual city is a maze of deep tunnels, storehouses, dungeons, and corridors that have remained entirely proof against invasion for eons upon eons. Others (perhaps less prone to exaggeration, perhaps less willing to see the truth) claim that the dabus’ supposed warrens are no deeper than fox dens or slaad nests: shallow diggings that are simply refuges for the dabus. The darkest rumors claim that the dabus wish to restore the pristine state the city exhibited before other races traveled the planes, when the Cage was entirely under the dabus’ control.