The Secret Order of the Gate is a left-handed Legacy that worships the forbidden eleventh Exarch: The Gate that created the Abyss. The Secret Order believes in two paths of service: The Opening Way and the Warding Way. Openers summon Abyssal manifestations; Warders banish them. The perfect Gatekeeper performs both duties, but few can maintain the balance. They see too much. Their minds break in one direction or the other and they pick a side.
The Secret Order's inner doctrine speaks of the Serpent and the Tree. The Tree is the Fallen World, growing from filth up to the Supernal sky. The Celestial Ladder was inevitable; the Tree always climbs, using the smaller life within it to manifest that fecund desire. If it ever grew too tall, it would envelop the heavens in its unclean tendrils and choke existence to death. It must be pruned, dismembered, and hurt from time to time. The Exarchs did it when they cut down the Celestial Ladder, and the Gate does it still, with the blackest, sharpest blade of all: The Abyss. That's the anti-reality's purpose. It's the Serpent, Niddhog gnawing at the World Tree -- but it too must be restrained. The Serpent and the Tree live in symbiosis. Unbeing seeks being, and devours it. Being fills the void until it's either stopped, or suffocates.
The Order has access to a semi-realm, in the utter proximity of the Abyss, that they call the Place Between. This is where the Gate dwells, and some Gatekeepers even believe that he is the realm. The Place Between looks like the material plane area from which a Gatekeeper enters, except that everything is twisted, rotting and corroded. New buildings are ruins covered in thickly-scrawled sigils. The sun is pale and the stars are red, arranged in unknown constellations. The Abyss manifests freely and its vile influence penetrates the whole landscape.
The Parametric Language is the Gate's occult tongue. The Secret Order believes the Lonely Exarch used it to command the Abyss to open, and tame the newly-Fallen World -- and to banish it, so that the fallen World would survive. It is a language that gives comprehensible structures to the Abyss' anti-entities. It's a tongue of negatives, that defines things based on what they are not, providing a general conceptual shape -- even a prison -- for Abyssal entities.