Donut
From Threesong
Above the Outlands, the Great Devourer, Homer, a cosmos-spanning serpent and one of the forms of Asmodeus, is frozen in that historic apocalyptic moment where he bit into the city of Sigil after breaking the Spire and initiating the Apocalypse. Homer's contemporary nickname is, of course, a reference to Sigil: A donut he is biting.
Much of the power and contents of the old Abyss are inside of Homer, and the inside of Homer is understood to somehow be outside the skin of the universe, beyond the boundary of the end of time, with a slimy demonic layer between that skin and the Far Realms.