The relationship between devils and demons is well understood through the lore of the Blood War. Is there a similar relationship between demons and aberrations from the Far Realm? Is there a relationship in Forgotten Realms canon that would serve as a backdrop or scaffolding to support interaction between the two groups in a campaign (e.g Descent into Avernus)?
There is no direct relation between the Far Realm and the demons. Far Realm is indeed far, it is not part of the Great Wheel, and can be considered to be outside of the regular cosmology. The entities of the Far Realm are foreign, while the denizens of the Abyss are still the embodiments of the chaos and evil within the beings that inhabit the regular multiverse.
However, if you want to put some connection between aberrations and demons in the Forgotten Realms setting, you might consider using the CE deity Ghaunadaur.
- Ghaunadaur is the god of abominations. 2e Demihuman Deities states:
It is venerated by the largest slimes, oozes, slugs, and other crawling things - some of which are said to possess intelligence, albeit alien.
- He was one of the deities who granted access to the madness domain in the earlier editions.
- According to the 2e Demihuman Deities (page 18), he pretended to be the demon lord Juiblex in the Realmspace. Hence it would be reasonable to think that he might employ demons. Speaking of Juiblex, according to the Demonomicon of Iggwilv (Dungeon magazine issue 188, March 2011):
Juiblex is inscrutable, strange, and alien, even by demonic standards.
- Finally, according to the 4e FR Campaign Guide (page 64):
The greater god Ghaunadaur is the mightiest of all entities that are thought to be touched by the Far Realm.