After the Great Silencing four great cycles ago, Planar travel was no longer an option and with it, Sigil was closed off, the old Gate towns (ie Bedlam) no longer had quite the appeal. Those in power who happened to be in the Outlands at the time searched for methods of understanding the situation at hand. A council of mages, Guild and unaligned alike, was called to the Great hall carved into the base of Yggdrasil. Those with more historical recolection brought up a similar situation thousands of great cycles ago when the planes had shifted. Some blamed the Baatezu as the stemmed tide of Tanar'ri would be a death knell for their side in the Blood War. Some Said that the gods had died or abandoned the planes.
Many light cycles into the conclave, A young Ogre Magi wearing a mask with nine eyes came throught the gates dragging a succubus by chords of silver, hemp and cold iron. After uttering words in what sounded like a crude combination of draconic and Terran, She plunged an obsidian dagger into the heart of the demon. She used the blood to draw an eye upon the ground around the fresh corpse. The soil sunk below the sign and a portal to the void was opened and entered by the young magi.
It is said that she has reappeared since, abstaining from the politics of the realm to recruit those she deems worthy of the knowledge. Her organisation is known as the Guild of Doors to outsiders. The true name is spoken only in draconic.
For the right price, it is said that you can contact the guild of Doors to petition them for travel. Acceptance is not guaranteed, but the advantages of free travel is undeniable.
Members of the Guild of Doors are known to wear flesh concealing robes, gloves and masks (or whatever is analogous to their species).