The Mad Bomber of Sigil
Basically, while the PCs are traveling along through Sigil, they're nearly blown to pieces by an explosion that takes down a nearby building. Apparently, someone has learned how to put Fireball spells in little packages. The PCs may give chase for the most likely suspect of the bombing (i.e. the guy that's grinning laughingly at it) but if they catch him alive, the guy kills himself via a glyph spell tattooed on his tongue.
A group calling itself "The Eso-terrorists of Aoskar" will then send a message to the City Guard that claims responsibility for the attacks.
Later, the PCs are given a summons by the Factol of the Mind's Eye who takes them through a portal to their new Headquarters. There, they meet Ombidas who informs them the Mind's Eye is getting a lot of flack for the terrorist attacks. He thinks its ridiculous, because quite a few Mind's Eye folk are dying in it. While he's conducting a public inquiry with his own people, he's interested in getting some outside help because people aren't answering his people.
Afterwards, the PCs will be contacted by Umbra whom is equally interested in stopping the wave of bombings that are affecting Sigil. What she'll reveal is that a member of the Illuminated Eye was caught in every single blast that has so far been conducted.
In truth, the PCs will find out that the Bombings are being conducted by an insane Athar that's assembled a group of dis-affected Doomguard, Revolutionary League, and Sodkillers around himself to throw mud on the followers of Aoskar's reputation. He's also using the attacks to kill a large number of religious Sigilites who just happen to be caught up in the blasts (The City Guard has missed the connection because they're a bunch of idiots).
Ultimately, the final attack will be conducted by the Mad Bomber against the Aoskarites at a rally that Aola is conducting in the Shattered Temple. The Lady of Pain will make an appearance at the rally after Winton's death but conspicuously spare the Aoskar worshiper's lives (for reasons that, of course, remain mysterious).
Winton Plaguemort
Chaotic Evil Elf Male [Prime] 9th Level Fighter/2nd Level Cleric of Demogorgon
Str 16
Dex 16
Con 14
Int 15
Wis 12
Chr 13
Appearance: Winton Plaguemort is an unusual sight in that he's a fat elf. He's also rather distinctive in that he's got a claw demon claw scar across the left side of his fight. Despite the nature of his physical condition, he's an excellent fighter and surprisingly spry for a man his size. Nevertheless, it's difficult to forget a figure as memorable as Winton Plaguemort.
Personality: The words 'child killing murderous fanatic' about summarize Winton Plaguemort. He's a ranting fool that drools out of the corner of his mouth, spews countless blasphemies against the gods, and talks to himself. Winton is a coward but frequently overestimates his ability to take on opponents, so he might end up fighting to death despite his unwillingness to die for his cause.
Background: Winton Plaguemort, not his real name, claims to be a 'Dark Elf' from some planet called Krynn. This is despite the fact his skin is white as ivory. Nevertheless, if you listen to his sob story then apparently his goddess was killed by an elven prince. Never mind the fact this goddess sounds suspiciously like Tiamat or no god has ever been killed by a mortal (at least without help). Winton certainly believes it.
After this 'Takhasis' girl's death, he abandoned his old name when he fled to the Outer Planes to cope with his tremendous loss. Eventually, he ended up in the gate town of Plaguemort and took a new name before hearing of the Athar Faction. The idea that the powers were frauds was somehow comforting to the Clueless and he swiftly joined up to become a crusader for the Defiers.
Winton was promptly kicked out of the Faction when he revealed his grand plan to go forth through the planes and kill every single priest that he could lay his hands on. The fact that he intended to ally with demons from the Abyss to do this is something that didn't help his case. Winton bares a grudge against his old Faction but still wears its symbol despite having been kicked out.
Winton is wanted for murders in a dozen Gate Towns and twice as many Prime Material worlds. He's mostly targeted the priests and holy men of minor gods or ones that are unlikely to fight back like those of Eldath the Goddess of Peace or Arrawai the Farmer's Goddess.
Recently, Winton has heard some tavern whispers about a revived cult of Aoskar. Figuring that the easiest type of god to pick on the followers of is a dead god, Winton has decided to create a scapegoat for his many crimes. Assembling a group of murderous scum around him, not difficult to find in Sigil, he's begun a series of terrorist attacks on the religious of Sigil.
Winton doesn't intend to stay very long in Sigil after he murders most of Aoskar's following here but he figures that he can strike a memorable blow against the powers while he's at it. Certainly, the body count is going to be conspicuously high even for him.
Motivation: Winton Plaguemort is obsessed with killing off as many Clerics and Holy men as possible. The only reason he's hunting Aoskar worshipers in particular is because of the fact that they seem to be less likely to arouse the wrath of the city than others. Winton is a seriously sick puppy and will eventually be forced to flee Sigil or meet justice at the hands of someone who tracks down his massive body-count.