Greg Keyes specified that he referred to a fan compilation of names previously used in the series when coming up with names for his characters. As the name Attrebus was used exclusively in Redguard before the novels, it is likely the name's inclusion here wittingly or unwittingly influenced the forename of Attrebus Mede.
The sword Umbra first appeared in Morrowind, and was given a backstory in the book Tamrielic Lore, which notes Naenra Waerr as the sword's progenitor. Umbra is the central MacGuffin of the novel; it is explained that the entity within the sword was split from Clavicus Vile by the witch Naenra Waerr. Umbra's escape from Vile resulted in the subsequent creation of Umbriel.
The island of Solstheim north of Morrowind first appeared in this expansion. It is featured throughout several passages of Lord of Souls.
Lord of Souls references a "new" Frost Giant living in the Moesrings of northern Solstheim. This is a reference to the death of Karstaag, the island's previous Frost Giant ruler, at the end of Bloodmoon.
Radiant Conversation dialogue in Oblivion mentions that: "Many of the Argonians say they have been called back to Black Marsh. They disappear into the swamps and are never seen again." This line is greatly expanded upon and given context in the novels.
Radiant Conversation dialogue in Oblivion mentions that: "In Morrowind, they say the man-god Vivec has gone missing. Some believe he has been taken by the daedra." While Vivec's current status is not addressed, the consequences of his disappearance are given context, eventually resulting in the fall of Baar Dau and the Red Year. This chain of events was used as the main backstory to the novels.
Sul and Attrebus's astral travel to cross the world faster by slipping into and out of Oblivion Realms within the penultimate passages of The Infernal City is similar to the interdimensional travel presented in The Doors of Oblivion book from Oblivion.