Spire

The Spire in the center of the Outlands is a tall, narrow mountain, or a monolith, posessed of a strange power of Balance that defies the powers of the gods themselves.

It is well-known, and has been proven, that the Spire is infinitely tall. It literally stretches into the sky forever, with no limit. It has also been proven that the Spire has a top. You can even see it on a clear day, with a gray torus hovering above it, just slightly above Infinity. This "Spire paradox" is widely cited as evidence of the strange and contradictory nature of the Outer Planes: space and distance aren't what they're cracked up to be.

The closer one gets to the Spire, the weaker magic, psionics, and other powers grow. Eventually, mundane poisons cease to function, and then even deities are reduced to the same level as mortals. For this reason, pantheons sometimes meet at the Spire's base, where all of their powers are equal and none can get their way through pure force.

In the Spire itself (and above it, where Sigil is believed to be), everything should be balanced out of existence. Time, space, energy, matter, and even thought should go to the same place magic presumedly goes. That makes Sigil, where magic works fine and everything seems to more or less exist and even think a little, perhaps the greatest paradox of all.