Aoskar is (or rather, was) a god of portals (doorways linking the Planes of Reality) in the Planescape expansion of the AD&D universe. Being a deity of portals, he was a tremendously important figure to the planewalkers of old; paying favor to Aoskar was done for luck before many a sojourn into the Planes. It is not unsurprising, therefore, that Aoskar's power and priesthood centered in the city of Sigil, that nexus of planar travel. The Great Temple of Doors was erected in the Lower Ward in his honor.
Aoskar's power and influence grew and grew over the years, such that his name came to be more recognizable even than that of the Lady of Pain. People all over the Cage hailed him as the city's master. The Lady permitted even this hubris for reasons known only to her - until Fell the dabus abandoned her to join the priesthood of Aoskar.
The Lady's response was instant and devastating. In her wrath, she smashed his Temple along with a significant chunk of the city around it (unheeding of the priests and worshipers within, or perhaps intending their destruction as well). The god himself met total destruction at her blades; she threw his flayed body into the Astral Plane where it floats to this day, neither alive nor dead. Worship of Aoskar was forbidden in Sigil, and those who disobey this edict tend to wind up dead or Mazed.
The Shattered Temple (as the temple's ruins are now called) houses the Athar faction, who use the story of Aoskar's destruction as evidence for their claim that the so-called "gods" are entirely fraudulent.
Rumor has it that there are those who would see Aoskar revived, even in defiance of the Lady of Pain. These suicidal individuals are not the subject of frequent discussion. Fell, the dabus whose daring brought about the destruction of the very god he worshipped, still lives and is still able to cast priestly spells... and that power has to come from somewhere. Is Aoskar not as dead as one might think?