The Phantom Plane is catalogued as the 7th layer of the+Abyss by the Fraternity of Order. The Tanar'ri call it Kearackinin.
All portals to the Layer were sealed by its ruler, Sess'innek. It was discovered many centuries ago when a wizard calling himself Mordenkainen managed to temporarily break the seals.
Sources: Planes of Chaos, Fiendish Codex I
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