Hey, if you don't mind me saying so, I think your Metatron is fairly underpowered for his station. I just pulled up one of my own First Ones to compare (who is only CR 374) and she was substantially stronger (which is a bit of an understatement).
I think the problem lies in copying Sandalphon's chasis. Meta's artifacts just don't add up to what he deserves, he's likely missing a couple 100 points of ECL equivalence of abilities from them.
I'd also suggest looking at my
Immortals Metamagic abilities, as well as giving him way more than a measly 12 AMC feats.
And, if you're interested, I'm attaching the stats for my First One, Ysrahl, that I used for comparison. She is, admittedly, the mightiest of my First Ones, but then, Meta should be in the vicinity, no?
I agree that statistically he is not the best HP wise or for base damage potential, I have a couple of stronger, physically, immortals. Particularly a strength diety named Queller, and a 'chosen one' type called Solaris with insane magic and ghost/ dragon based powers and dreams of becoming a Time Lord.
Metatron's big power isn't his ability to hit a rediculous amount of times per round with a pretty sizable attack bonus. Granted a 700+ can easily hit a Great Wyrm Nexus Dragon and even a Neutronium Golem, 72 wing slaps, each doing divine damage, with his aura, a free gaze attack that works 4 times a round with no save, and his touch ability coupled with his sword is still pretty good as far as damage potential goes, I think, without being over the top.
I deliberately made his powers more subtle in nature rather than just being a powerhouse because if you look in the Immortals Handbook Bestiary near the beginning, forgive me I don't have it handy, it talks about how when demons crossed the dimensional rift or whatever it's called they basically trashed the Angelic Host, meaning to me, that if the Seraphim, Ophanim, and Cherubim are their heavy hitters, and they weren't able to really stop them, if Metatron was as powerful of a combatant as you seem to imagine, and had all these combat oriented powers, nevermind that the angels at this point had no real reason to have insane combat prowess as it states angels were the only beings in the outer planes, wouldn't he just be able to insta-pwn the entirety of the encroaching horde. If his powers with damage based and destructive in nature, why wouldn't he just one-shot them? My vision of him was to make him able to
force dialogue vice just pee-pee slap any who crossed him, but at the same time still give him enough options to put someone down if needed.
It also states in the Immortals handbook as well that Sandalphon is Metatron's Aspect, slightly scewing that to make a manifestation doesn't seem unreasonable, in my opinion.
About his artifacts, I agree that statistically they seem underpowered, but if you look closely at the powers and how they work in union together and with his abilities, it is set up that way to make him in essense invulnerable.
For example: Epic spells can't effect him, actually they can be directed in any direction he sees fit, he is surrounded by dead magic meaning that even if it could get around his artifact's powers it still needs to contend with that. His robes revisit damage upon all who actually sucessfully hit him and thier allies, his learned ability immunity makes all supernatural abilities and below one-trick ponies, but ultimately though his big power is Edification.
That's the beauty of his design; those who enter his aura are immediately changed to be a good alignment with no save, his Holy Messiah power makes any hostile actions from that character then impossible. Therefore there is no reason for him to be the biggest gun on the battlefield when he can use diplomacy to stop the battle alltogether before it even starts.
Horde of bloodthirsty Maskim coming to tear the universe to ribbons? Poof, now a horde of shiny Pala-skim wanting to save the universe.
Also his ability to summon an army of angels to his side has it's merits too...
That said however, I do really appreciate the input, and I honestly expected somone to have this reaction. It's not unreasonable, all one has to do is look at Death directly above and one can see the statistical differences particularly in the HP category, though Death has almost double the Quint.
I am still of the mind though that given his powers I don't think his CR is off, perhaps the auto metamagic capacity. I blew all his feats into anyfeats for simpliciy's sake. That being said, tweak him for your use if he isn't to your liking. Those anyfeats could be changed instantly into 72 extra amc feats on a whim if you really wanted to.
I like your First One btw, would you have any objections to my using it in an upcoming campaign?
Sorry for the edit:
I was looking over Ysrahl's powers and I was curious where you got 'Living Magic, Antideluvian Ancestry' and a few other powers from? The link you sent me was broken so perhaps it was from there? I am curious if there are stats for these abilities or data, To gain Dead Zone you have to take a Divine Ability tree to even gain it's use and have 130+ int. Speaking of stats I was also curious how her stats are so high, why her saves appear to be doubled, and why she has so many extra cosmic powers? It seems like she has alot of arbitrary boosts, don't get me wrong, I'm not saying that's a bad thing. I have plenty of boosted characters, primarily NPC monsters like Anomalies and such. I was just wondering where they are from? If you are 'fluffing' a character for gaming purposes that's cool, go crazy, but don't compare a fluffed character to a reasonably legit one and say 'Hey look at this flaw!' That's kind of crap.
That said, I'm sure it's simply an oversight on my part.
Also the HP seems to be too high, in the Ascention guide on page 40 it says that First Ones gain d100's for HD and have 8x maximum HP/ die. I don't think that applies to HP bonuses from high con or other abilities. I mean I don't think toughness would give you x8 it's usual due to Max HP boosts for being a First One. Perhaps I'm wrong. UK, would you mind correcting me/us if possible.
I did screw up however on the HP part on my side, looking back over the First Choir Angels, they always gain x2 total HP, so he would actually gain x2 of what his current HP is. I will fix that immediately.
As long as you're okay with her metamagic abilities, go ahead and use her. She's dead in my campaign anyway (was eaten (literally) by one of her brothers ^_^). 
