Sauron, the perverter, violator, and who swelled with lust and greed, ever greater, with the more power he allured-subverted, held, incorporated, and violated.
My thoughts:
Secretly, as Annatar, quested to develop a violating version of life extension. Use of the Rings for Mortals meant the men enslaved were 'life syphons', incarnate. Their aura grew in Sauraon's image, and to behold them as Nazgul or hear them was a life-threatening event. Stolen life force. The fear felt around them was instinct governed by the predatory dimensions of human form. I see them as great 'energy sinks' drawing in Living Energy to transmit to master - swelling Sauron's filth, greed, lust and power as the Nazgul engorged on life.
They had a spiritually fortified flesh (albeit, by an 'un' flow, perhaps of Ea running in some variation and inverse). I recall reading somewhere the moment of transition as a Ring bearer of being 'naked' before Sauron as mortal sinew was blasted or stripped of natural form, and imbued with whatever metaphysical joo joo Sauron did.
The flesh could be hued - by Hobbit and Woman. Not sure why or what significance this implied about Sauronic violations.
I've imagined that using a Ring, pre transition, was about greed, sexual perversity, (I assume the so called Lords who had the rings attracted 'bad boi sex' and concubines and such
) And that greed - lust - the primary reinforcers were the reward as the man disowned love and care, by increment. The flesh fortified by increasing flow of metaphysical form of Sauronic Mind (he was a torturer, a sadist, a violator of life, a seducer and a slave lord, above all else).