The Peril of Sigil

The Outlands have many towns, but none come close to Sigil, the city at the top of the Spire. The Spire is an unassailable mountain at the center of everything. Recently, Sigil has been cut off. No portals to it work and nothing has come out of the city. This is where adventurers come in. The Spire has a hole at the base known as the Underspire. Portals to other planes dot this interior. The Underspire is thought to climb all the way to Sigil, but all attempts have failed. Only the most daring and foolhardy challenge to not only pass through the Underspire, but also reopen Sigil from the unknown cause.

The Base

The Underspire's first level somewhat resembles the Underdark of many worlds. It has many similar creatures and ecosystems. It is known as the Base. Nightcrag is a city on this level that allows visitors. While murder is illegal and heavily enforced, fighting is allowed and erupts constantly.

Levels of the Underspire and the Astral Stairway

As explorers climb the Underspire, planar energies leak through, creating different levels that are both part of their parent plane and part of the Underspire. The only way to ascend through the levels is to find the Astral Stairway, a passage between planes that specifically connects two levels of the Underspire. Every other attempt fails, as most levels are a near endless landscape, with a sky and ground. Trying to fly or dig to another level is fruitless, as all resources run out before reaching the next level.

Caiphon

Caiphon is the sixth level of the Underspire. It defies explanation as it changes forms and does not follow any understanding of space. Turning around a corner and then returning may leave a person in a completely new area. The Abberations that inhabit the level navigate flawlessly, however. This level is the reason for the closing of Sigil. The Chained God, Tharizdun, has bent the will of its cultists to create this level. By infecting the rest of the Spire, Tharizdun would destroy it, thereby freeing itself. Sigil is under seige by these cultists, who have trapped the Lady of Pain using an ancient artifact known as the Shard of the End. They now are conducting a ritual to destroy Sigil, allowing the Spire to become further infected. If Tharizdun is unleashed, it will seek to obliterate everything, until it (and its cultists hope, his followers) is the only thing remaining in the nothingness.