Yeah, I know. The devil. Edgar (as Poor Tom) says those words when he sees his father. The latter is remembered of Edgar, as he tells us later; the former says: "The Prince Of Darkness is a gentleman. Modo is called, and Mahu"
But retrospectively he also seems to be, or have been, Gloucester.
"The dark and vicious place where thee he got/cost him his eyes". That place is female genitalia. And in the play we have Lear saying 'there's hell, there's darkness'. That's what female genitalia is in this play.
Edgar, as Poor Tom, says:
[I am] a serving-man, proud in heart and mind; that curled
my hair; wore gloves in my cap; served the lust of
my mistress' heart, and did the act of darkness with
her[...]
Just as Gloucester is right without knowing it when seeing Poor Tom he's remembered of Edgar, maybe Edgar, seeing his father, speaks the truth about him or about who he had been as a young man, also without knowing it.
True, the Prince of Darkness is evil, a villain, and Gloucester is not. (Edgar was not the wretched Poor Tom either, but he was wretched enough)
But far as lust and brothels are concerned, though, the Duke/Prince Of Darkness overlap is notable - and the mere fact of sexual division, of sex, is tragic enough in this play, as it relates to the ideas of Nature and to 'conception' (a shakespearean pun, and the Mother/Hysterica Passio it's related to this too)
Maybe these two Dukes were somewhat similar (and Vincentio is repeatedly called prince in the play)
LUCIO
Why, what a ruthless thing is this in him, for the
rebellion of a codpiece to take away the life of a
man! Would the duke that is absent have done this?
Ere he would have hanged a man for the getting a
hundred bastards, he would have paid for the nursing
a thousand: he had some feeling of the sport: he
knew the service, and that instructed him to mercy.
GLOUCESTER
But I have, sir, a son by order of law, some year
elder than this, who yet is no dearer in my account:
though this knave came something saucily into the
world before he was sent for, yet was his mother
fair; there was good sport at his making, and the
whoreson must be acknowledged.