I wanted to write this myself, but I figured everyone should look over the details for what they think of it.
The idea being that there's a bunch of anger and resentment over the Shattered Temple being knocked down and paved over to build the Temple of Hades. A group of Athar extremists get themselves a Baatorian bomb to blow up the Temple.
The Temple is currently run by Friar Murlov Garianis (NE Prime Human Male Priest of Hades 19), a Proxy of Hades who has a serious beef with the Athar. Long ago, Murlov lost his son and heir the position of Patriarch of his homeworld's Chapter of the Church of Hades thanks to the meddling of a Athar while sending him to explore the Outer Planes in preparation for his ascension. Thus, it was poetic justice in his mind to use his immense wealth (Hades being the god of wealth in addition to the Underworld) to bribe his way into ownership of the Temple of Aoskar. The House of Records being looted and burned during this time helped ease along the tradition and the Athar weren't exactly rolling in jink at the time.
Likewise, Vargas Garianis (CN Planar Human Wizard 7) has an abiding hatred of his ancestor. A member of the more militant faction of the Athar, he's been spending a great deal of his time amongst the Revolutionary League since the Faction War. They've half-convinced him to jump Factions but he's more interested in incorporating the Revolutionary League's violent methods into a covert war on the gods. A symbolic strike that blows up the newly created largest Temple in Sigil (over the Athar's headquarters no less) would do wonder for his and his faction's reputation.
[Specifically: A lot of people would move their opinion of the Athar from deluded philosophers to dangerous terrorists]
Depending on where their sympathies lay, one of the two will approach the PCs to act as either guards or the agents for blowing up the Temple of Hades. If the PCs choose not to intervene then Vargas and his group chooses Midnight Service for the bombing and close to a thousand innocent parishioners are killed. If the Pcs choose to plant the bomb then they can choose to set it when only Murlov and most of his NE subordinate are alone in the Temple. A full state write-up will be necessary for Aeneas as it's possible, but not necessary, that the PCs could be spotted by him. Also, Vargas and his team will need to be stated up as they'll handle the actual attack if the PCs are guards (the priests in the middle of mass and all of their spells prepared for Healing and other miracles than battle due to the grand re-opening).
If the Temple is destroyed then it will curiously look almost exactly like the Athar's previous Temple before and the region's history in Sigil as a cursed zone will prevent anyone from attempting to purchase it again. They will once more start calling it the "Shattered Temple."
If the Pcs blow it up when it's not full of parishioners: The Athar will be largely supportive of their efforts and congratulatory since Aeneas has been a pain in their ass for years. Furthermore, the Athar will be able to successfully sue the city to regain ownership of the Temple grounds.
The death of a Proxy also wins them much respect with Athar in general, though Hades will want some form of compensation for the deed (perhaps 50,000 GP or a quest. Oddly, he's less mad than he might be because he found the idea of courting the Ladies wrath like this to be troubling)
If Vargas or the Pcs blow it up when it's full: Then the Athar left in Sigil will be openly attacked, their shops looted, and quite a few strung up in show trials. The city stops just short of banning them in the city outright. Also, the Athar's reputation takes a nosedive with many supporters.
Shockingly, the militant faction gains considerable ground within the Athar though and the Revolutionary League becomes allied with them (to the Old Guard's horror). The PCs will only be welcomed by the militant faction.
If the PCs successfully protect the Temple: They are rewarded by Aeneas with a substantial amount of wealth (perhaps 10,000 gold to divide amongst themselves) plus a magical item. Aeneas will also take personal custody of Vargas if he's taken alive and intends to brainwash him into a servant of Hades.
Over the next year, the Temple of Hades suffers numerous accidents and bizarre incidents that cause Temple attendance to drop off sharply. Eventually, the roof collapses and Aeneas is crushed underneath. The Temple is abandoned shortly thereafter.
[Another option is that it goes on to become one of Sigil's most beloved icons]
So, questions, criticisms? Suggestions?