Mandos in Tolkien is very different from the Mandos that we have in our lore. In Tolkien Mandos is never corrupted, he just pronounces people's doom at the bidding of Manwe, which are also the will of Eru. He also judged the elves that died and they would be kept in his halls. He even kept Morgoth chained up in a castle called Lumbi at the guard of his hound Gorgumoth. Mandos did not have to have a side gig as owning a Law Firm (though we love this in him in game).

Mandos was described as "a dark figure with a loud voice, solemn and terrible." He was stern and dispassionate and never forgot a thing. He was moved to pity only once when Luthien sang of the grief she and Beren had experienced during their life in Beleriand. He got approval from Manwe for them so they could live a second life on Middle-Earth as mortals.
Reveal Spoiler

Good synopsis of Mandos, Damen takes his liberties in terms of CZ. Nothing is directly copied, merely inspired.