That was almost trivially easy, but there you have it. I adapted my existing DARK•HERITAGE m20 rules to this new setting by copying and pasting, mostly, the rules, editing out any specific references to the setting, putting it on fewer pages, and mixing up the races a bit. In fact, I used the race rules for my Star Wars m20 game, but renamed some of the races to be more in common with what you see in fantasy; i.e., humans, elves, dwarves, halflings, orcs and cursed (by which you can assume that I mean tieflings, basically. Tiefling as a word is product identity for Wizards of the Coast, so I have to make do with the concept without using the word. The concept is, of course, as old as the hills and therefore public domain; Merlin himself is said to have been a changeling, for instance.)
As in D&D, humans are the most versatile, with a bonus to all skills. Halflings are stealthy and dexterous, orcs are strong, elves are learned, and dwarves are hardy. The cursed are a bit more nuanced, with a few minor bonuses, including to their DEX score, their Physical skill, and their Knowledge skill, to represent intuition that is their heritage from a fiendish ancestor. Although arguably, their stats look more like what you'd expect from a d20 elf than a d20 tiefling. Oh, well.
Because I'm using the Star Wars race system, of course, you can tweak any of the stats to your taste, or even create any racial profile (from a mechanics perspective, at least) that you wish. I don't really recommend that you do so, and even less do I recommend that you people your PC groups with odd and bizarre races that you've created that don't have any place in the setting. However... well, everyone likes options, even if you don't plan on necessarily using most of them.
EDIT: I'd also like to add the Shadow Sword class to the line-up, now that I think of it. It seems entirely appropriate for this setting. Plus, here's a picture of what could be a Cursed Shadow Sword. Who wouldn't want to play this guy?
EDIT 2: I noticed, as I was going through the spells, that I have already named my Tar-Baphon after all; or at least I have a spell specifically named after what is meant to be the most iconic, legendary necromancer in the history of everything. However, I deliberately hearkened back to Nagash with my 5th level spell, Command of Naggazz. I could always edit the name of the spell, but most likely I'll use that. I've always thought Nagash was a great name. I don't want to tread too heavily on it, but I'm happier more or less mimicking it than many other options, honestly.
Next up for CULT OF UNDEATH is to start developing the setting. That won't be trivially easy, and I can't just cut and paste it with a few minor edits, so that will almost certainly be a slower-going process.