I've been wanting to take an adventuring party to Sigil for ages. It's a treat I've been saving up for myself because the place is so very bizarre. Anything and everything can happen there.
For those not in the know, Sigil is an extraplanar crossing point between all the known and unknown worlds. It is a kind of urban Wood between the Worlds with fewer guinea pigs and a lot less restful. The Lady of Pain is no Aslan.
Despite my longing to go there, I've held off. For one thing the place is dangerous and for another, it's so crammed with potential adventure that it just feels overwhelming for GM and players alike unless there is an actual purpose. Nothing worse than setting up something awesome and then to be boggled by choice. Paralysis results.
Now, however, I have the perfect party. The Tombs I group need information. The person they need that information from is in Sigil.
They still need to get there of course, which will an adventure in itself. While it's true that one can reach Sigil accidentally through a broom cupboard, that doens't feel quite heroic enough. Plus, they need another level under their belts before they can sensibly track down their contact without being obliterated.