Neth the Plane that Lives (Great Old One), from the Planescape Campaign Setting. It's quite curious about the multiverse, and occasionally consumes living beings to absorb their knowledge. It also has sends out parts of itself in the form of Nethlings, who are shapeshifting beings of not well-formed flesh and were a PC race. A Warlock of Neth could be a Nethling themselves, and might have been someone who was consumed by the living plane.
Apomps the Three-sided One (Fiend), is the lord of the Gehreleths/Demodands from Carceri, Apomps was once a Baernaloth that was exiled, and now seeks to destroy the other Baernaloths and their Yugoloth creations.
Supreme General Absalom (Celestial), the supreme general of the Grand Celestial Host of the Asuras, an army of wrathful and fiery Angels from the Chaotic upper planes. He or many of the other lesser Asuras would probably be quite in imbuing mortals with the ability to scourge their enemies as the Asuras as very much the rivals of the Devas and don't tend to serve any Gods.
The Silver Flame (Celestial), while there certainly are Clerics and Paladins of the Silver Flame in Eberron, it's more of an energy source. I remember there being some class or prestige class for someone who channeled the Silver Flame. In 5e that's very much a Celestial Pact Warlock.
Ravana (Fiendish) from Hindu mythology Ravana is the King of the Rakshasas, he has Ten heads and seeks to dominate the Gods.
The various Celestial Lords which include:
The Archon rulers Barachiel, Domiel, Erathaol the Seer, Pistis Sophia, Raziel, Sealtiel the Defende and Zaphkiel of the Celestial Hebdomad.
The Guardinals champions: Talisid the Celestail Lion, Bharrai the Great Bear, Kharash the Stalker, Manath the Horned Duke, Sathia the Sky Duchess and Vhara Duchess of the Field.
The Eladrin Rulers who could be either Fey Pact or Celestial Pact: Morwel the Queen of Stars, Faerinaal the Queen's Consort and Gwynharwyf the Whirling Fury.