First off - thank you, pal! I really appreciate the support! Now, down to the constructive critique!
I've never seen Asmo as much of a fighter. Rex is designed to charm-lock parties. I've done that at lower levels and it is devastating; giving that power to Asmo only seemed natural. In particular, I thought the idea of adding character-bending flaws to his enemies probably dissuades all but the most certain assaults. No one wants to pick up any of those.
Also, I think the DCs and how quickly Asmodeus can charm an entire swathe of PCs keeps his CR from feeling inflated. Rex basically blinds the party every round unless they want to chance the DC 23 Charisma save, which effectively ups Asmo's AC to 24 due to disadvantage. They don't gain immunity to this effect like they do against dragon fear, so the PCs get hammered by Rex every round. If they chance it and didn't roll at least an 18 total on their save, the team has to burn action economy to remove curse or that PC is gone for the fight - Asmo just pockets them.
I do agree that Rank could have been listed as a Lair Action very easily, so maybe that's something to think about for a v.2 - I'm glad you like the ones he has, tho! Stacking the sickly halo and Father is a nasty bit of business. Asmo should have Ignan, so I know I'm going back to the statblock at some point... Typo on Tyranny, noted.
And yeah, artifacts are a bit of a slog. There are a lot of parts to chip through and you have to try to make it really stand out in a small burst of text. Mjolnir was fun to write, though.
Now, the Overlord. Yeah, he's supposed to be basically unbeatable. That's all based on the premises set forward in "Beyond CR 30", where I sort of blackbox the math on a CR 35 then backfill the rest of the CRs 31-34. I explain the tiers in the PDF, but in a nutshell there's enough in the books to make me think there's a tier of monsters beyond CR 30 that capture anything we'd classify as a "greater deity", or something on par with one.
Basically, a CR 35 has enough HP and does enough damage to be functionally invincible to anything you can build with the core rules, and probably a good deal beyond that (allowing for the sorts of power stretches that you see in games that are innocent of min/maxing but still not strictly by the book). Even if the DM lets you max out HP and get a full set of top-shelf +3 armor and weapons, the Overlord is basically guaranteed to 1-round the party. Stacking all the HP feats and gear won't matter, you won't last more than a round or two. No sort of +3 weapon will make the difference, the Overlord has 6600+ hit points. Almost beyond doubt, the Overlord is invincible to anything short of gods or PCs with a ton of boons. I still haven't done the math on how many, but it's probably a lot.
Anyway, only a collection of other lesser deities (CR 26-30) and/or greater deities (CR 31-35) can overpower a CR 35. Re-Horakhty could be overpowered by other deities in his tier, or even a boatload of deities from the tier below his (CR 26-30; i.e. beings like Demogorgon), but unless the PCs are immune to fire and radiant damage, he's probably going to clean up. If a PC party managed to beat Osiris (CR 34), I'd be surprised (but intrigued).
I'm psyched that you're checking stuff out, Will. I'll try to have Asmo updated soon - thanks for the red pen! Next up is the last official Pantheon, covering some of the Norse gods (Valkyries, Hel, Surtur, Thor, and Wotan). Then I'm thinking of tackling the Lords of the Nine, so that should tie in nicely with an Asmo update. Looking forward to any thoughts you have on other stuff!