Input asked

very quickly: no one can be summoned inside Sigil, you can enter only through portals. So the party should start somewhere else in the outer planes (the Abyss? the Outlands?). Maybe they have got to go to Sigil and actually enter a maze to rescue/kill/help some other important Lolth follower (maybe the one who started the mess). Maybe the Athar know the dark to reach that particular maze...

This is not a good setup. If the players don't know that there's going to be more after the ritual, they will be very displeased as the "sacrifice" proceeds; they will, of course, try to survive, and you will have to railroad them to keep their attempts from succeeding. You may not be able to get past this without difficulty.

As has been pointed out, no one can be summoned into or out of Sigil. Primes in particular cannot be summoned anywhere, at least under Planescape's original rules.

You might instead try being a little more open about the problem. A "foreign" priest shows up. He acts a little odd, but he flashes gold and seems to be looking for a few stout believers. He recruits the PCs if they're willing to travel to a "foreign land" to help out the faith, because the local hierarchy is in trouble and needs some help from abroad.

A possible motivation for the Lady's anger might be that the perpetual haze around Sigil has been growing thick with cobwebs. The city leaders are beginning to suspect that Lolth is attempting to capture the city by winding it about entirely from the outside, since deities cannot enter Sigil. Whether this would work is dubious, but it's a creative attempt, and it's certainly making life unpleasant for non-spider-worshippers. The dabus and the city watch have been going around sniffing out Lolth-worshippers, but Primes escape their detection devices, and so a few are recruited with orders to lay the blame for the plot on, say, Anansi or Mishka or some other spider-being.

Whether Lolth is [i]actually[/] behind the plot doesn't even need to come to the PCs' attention unless you want it to. Most likely the plot itself will fail when the Lady decides to will the haze cleared for a while, setting off an unusually bright, dry day with a long, clear view of the Outlands below. For the ending, what matters from the PCs' point of view on their duties is whether they managed to create enough doubt and point enough fingers at other spider gods' followers to take the heat off of Lolth's hierarchy for a while. If they fail and some high-ranking priests of Lolth get mazed, there's unlikely to be much reward coming their way.

For a twist, if it wasn't Lolth, it might have been ancient followers of Mishka the Wolf-Spider, seeking to turn Sigil into a cocoon from which he could re-emerge into the universe. That could uncover a whole campaign of plots, leading up to the Rod of Seven Parts.

Fair 'nough. What if the current Sigilian conflict spills over into the PCs' home instead? Instead of offering them money, the stranger priest is found on the trail coming in from a remote point, dead. The body and the nearby area has been defiled with the markings of a rival spider-deity like Anansi, but not robbed of several valuable items sanctified to Lolth. If there are good trackers in the group, let them trace his attacker back to the occasional portal that the priest and his rival both used. If not, have him be carrying a paper with orders to recruit several natives, and directions from the portal's location to the PCs' home, along with two-way instructions for use of the portal (it needs a key, which is why it wasn't noticed before).