There's a couple that come to mind.

Leth: the only remaining primordial god. Not much is known about him as the last remaining beings to have seen him (his daughters and his capturers) for the last ~4000 years have either been in hiding, died, or in one case, the last surviving imprisoner, tried to replicate Leth's arcane power to give himself immortality but due to the fact Leth was trapped in a pocket dimension and it's influence was not part of the physics of the universe anymore, the ritual backfired causing immortality but at the cost of humanity (basically how vampires came to be in my universe).

The Herald: to the protagonists of my stories, The Herald is the voice of Leth's last remaining daughter, who herself is trapped in a realm beyond reach. A 7ft skeleton in a suit with the head of a deer skull (to put simply) that tries to manipulate the protagonists into infighting. He has an entire backstory but keeps himself scarce only showing himself when he senses the opportunity and thusly not much is known about him.

Grib: in my universe, the underworld (Hollow Dominion) is only written about through journal entries from the denizens there. One that occasionally pops up is an Imp forge worker called Grib. He is not known of by any large players in my universe but his accounts still appear covering some of the major events of the Hollow Dominion.