I could swear I read somewhere in canonical Planescape material that another priest of Aoskar was mentioned besides Imendor (Aoskar's last priest) and Fell (Aoskar's proxy). (And no, I don't mean Aola from Planescape:Torment; this is another character found in the actual PS game books). Whoever it was, was given a name, but I can't figure out where I read it. This is driving my bone-box barmy... maybe it was his last lay worshiper. (P.S. I also don't mean Niloc or Origax... this was a deceased character!)
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I don't know if this is helpful to you but there's the Signer subgroup that I believe is called the Will of the One. They've secretly been trying to bring Aoskar back to life. The factol one of the ringleaders for this group.
I don't know how many of them actually worship(ed) Aoskar, though.
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