I mentioned earlier that Sentient Pest is very DS2-coded. A lot of the nightlords are Souls-themed, at least to an extent.
The Darkdrift Knight especially.
The new sovereign event has them adding Gwyndolin's arrow magic attacks, lightning 'miracles', and the upward thrusting spears associated with the Spears of the Church, and, of course, a big-ole Ms Marvel Manus arm. It has hair like Manus' arm, but no apparent mouth or teeth, and it has three fingers like the pus of man enemies. 'Darkdrift' is a souls concept that refers to something existing between planes of existence, something half-in and half-out of the material and ethereal planes that can act in either with the advantages of the other; it was a concept associated with Agdayne & the Undead Crypt and, later, Yuria & Londor.
He's described as having lost his arm when his companions attacked him from behind. Fulghor seemed to find the companions' betrayal of their gods more despair inducing than their betrayal of Fulghor himself. These gods were described as 'mightily radiant' and able to 'conceal themselves with thunder'. This would particularly seem to invoke the Warriors of Sunlight and the Sun's Firstborn, presumably named Grynn by the Menial. Serpentine Herald on Youtube has a great video breaking down the etymology Gwyn's lineage go watch it.
My thinking is that Fulghor is from very early or very very late in Lordran's history, possibly pre-ringed city. He would probably be considered a demon in souls parlance in the same way that the Sanctuary Guardian is a demon, but Fulghor's more Iz-ian features seem to be either better hidden or completely absent.
His spear/catalyst doesn't look like it was intended to be used as a melee armament; it looks like something that would be used by some kind of siege engine. He 'spiritcalls' the 'spear' into a twinblade during his special phase and reveals he can fly to an extent, but seems to need to pray and/or absorb a lot of sunlight to do so.