Rather obviously LA -0 as-is.

I'm kind of wondering what would be a sensible 21+RHD monster in a more normal CR range, to be perfectly honest. This is actually a pretty good starting point for such an exercise, given the innate no-cost (Ex) Flight and health recovery, right alongside ability score concentration and passive SR. The issue it faces in turning it into a PC is that the only scaling here is save DCs and Unholy Grace, and Undead RHD are extremely bad.

But a major thing that tends to be passed up in these threads is that all these monsters that have CR-appropriate AC with bloated hit dice get that AC on top of all their items. So you throw on your Epic armor as per usual and... Have +19 AC over a "blank" character in your niche, from the Natural Armor, Dexterity, and your bonus Charisma. So you literally roll over the die completely an entire extra time on your armor class, in the event you can match the other sources of armor class. And if not, well, you would most definitely be overpowered if that wasn't the case because you'd be guaranteed that any sensible encounter only hits you on a natural 20.

Similarly, the massive Strength bonuses translate to per-hit damage, meaning you inbuild 13 damage per hit, and the four Natural Attacks are usually superior to 20 pre-Epic BAB because of fewer iterative penalties. A standard character needs ~4d6 or 2d12 damage per hit from features to recoup that difference, and will ironically usually have fewer hits because this thing has a full +26 to attack rolls baseline and runs on Natural Attacks, where an actual Epic character has +23 BAB. Granted, getting an extra +3 to attack rolls from class features is so trivial Fighters can do it, but it's still an angle to work with.

The core problem is that, while its chassis is quite likely to be solid for its RHD-determined ECL, it doesn't do anywhere near enough with it. 15 ft. radius is terrible, needing to touch the ground is silly, and the damage doesn't really plug into anything. Rotting Touch being per-hit would do a solid amount to help, as would being in ratio healing... Like the Bleakborn all of three entries later that's CR 7 and 8 RHD. And, of course, things outside combat to do. Probably based on spawn, or Disintegration-like properties from decaying all sorts of random objects.