Howdy, howdy. I read through your Nephilim race and I like it. I'm not sure if this is posted somewhere else or if you've updated it since August- Anyways, I thought I could offer some help, more or less, in the technical jargon that the core books use and offer some suggestions to balance the race a little better. And if I come off as a dick, I apologize.
Firstly, Otherworldly Instincts, Detect Evil and Good is a 1st Level Spell. But instead of making it a once in a rest thing, you could word it "You can cast the Detect Evil and Good spell at will."
Um, for Child of Three Worlds, personally I think it would be fine just to call them humanoids, but instead toss in Magic Resistance because in most cases celestials and fiends have that trait in common.
Natural Weapon Master fits pretty well considering the Darksiders series. No problems here.
Same thing with Unwavering Will.
As for Horseman's Rage, I think it would be better if it were a bonus action, or perhaps as a reaction to taking damage. I like the damage and the subspecies damage type, I think you could do without reach/range bonus though. Also, I would suggest replacing the "attacks everything in front" with an Extra Attack. Mostly because, unless I'm misreading it, at Level 1 hitting five times is a little OP in my opinion.
I like Scion of War, maybe rewording the Weapons Enthusiast to "You are considered proficient in Intelligence(Investigation) checks and Wisdom(Medicine) checks to determine the cause of a wound or weapon damage and you add double your proficiency bonus to the check, instead of your normal proficiency bonus." Unless it's an outstanding circumstance, in most cases wounds and up close inspection use those two skills.
For Scion of Death, I would suggest a Wisdom score bonus instead of Dexterity. Dex still would work, but from taking example from the core books, in most cases, any race's subspecies never held the same ability score increase. And the whole, Death being the eldest of the Four Horsemen and all the cliche sayings about age, Death, and wisdom.
Also, I like Macabre Affinity. All it needs is technical clean up because the way it's worded means a Lich won't attack you. I would suggest something along the lines of "Undead with an Intelligence score of 6 or lower will not attack you unless provoked. Also, you have advantage of Charisma checks against Undead."
For Scion of Fury, go with the Dex score bonus. It ties in well with Acrobatics Pragmatist. For the feature itself though, maybe just a copy and paste of the Defensive Duelist feat would be simpler.
For Strife, I would make it a Charisma Score bonus, again to boost synergy with the feature. And maybe, "You have advantage on the first Charisma check you make against a creature." And then below it, have a copy and paste of the Silver-Tongued feat.
Although Absalom was featured in Darksiders 2, I don't think Scion of Absalom is necessary. If you were to keep it though, I think swapping Strength for Constitution bonus would be a better choice. Instead of the first part of the Corrupted Heart feature, I would give it poison damage immunity and poison condition immunity. I like the regain hit points for neutralizing poison in another creature, though I think 1d6+Constitution Mod would be a better formula because there isn't a spellcasting ability for the race as far as I can tell.
Alright, that wraps up my critique of your race. Overall, I like it a lot. I get bored and build DnD characters in my spare time. A friend of mine asked if I could build the Four Horsemen from Darksiders and I was struggling with finding a "good" Nephilim race. Yours had a sturdy foundation and held true to the Darksiders series. And once again, sorry if I came off as jerkish. Best of luck to you in your future Homebrew adventures.
