Sigil

"a sign, mark, or seal," mid-15c., from Late Latin sigillum, from Latin sigilla (neuter plural) "statuettes, little images, seal," diminutive of signum "identifying mark, sign" (see sign (n.)). In astrology, an occult device supposed to have great power (1650s).
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Sigil is a strange, Gothic, and often raucous metropolis that lies outside the rest of the Multiverse, inhabited by all manner of odd folk and ruled by the mysterious Lady of Pain and political parties —"philosophers with clubs", in more ways than one— called the Factions that Player Characters could join, all of them searching for the Truth behind the Multiverse. It is the hub of a vast network of magic portals that lead to many places, including: [...]tvtropes.org
Sigil, also known as the City of Doors or the Cage, is a nexus and planar metropolis. Every plane can be reached through portals from the inside of this torus-shaped plane, and the only way to come or go from Sigil is to use these portals. Sigil, the city, is on the inside of this hollow donut and is separate from everything in the multiverse, but connects everything. Trying to go to the outside of Sigil will bring you to nothing, literal nothing. If you wish to travel to a specific plane, Sigil can get you there… you just have to find the right portal and key.dumpstatadventures.com
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* Sign; Symbol; Hypersigil; Interesting idealism; American mythology; 
*U.S. mythology; Neo-mythology
* Sigil is a city in Planescape/Forgotten realms;
* AQ-753: a planar metropolis that leads everywhere