I've spent some more time reading through the document and properly thinking about the class, features and paths provided. This is all still just theorycrafting:
I agree with everything that was posted by Shadowy Reality as far as the base class Hitdice and ab/ac is compared to old Hexblade. Their math holds up.
The class gets 3 Pre Epic, and 3 Post Epic class feats to pick from.
I've mostly compared the paths to eachother and have thought about how their atrocities work in combination with what I think would be most builds for this.
First of all, it can't use UMD natively, but since its an arcane caster with up to 4th level spells, it can use most of the important wands without the need to take it. UMD would only really matter for using grimoires, but I can easily see a lot of people giving up 1 of their epic class feats to either get access to UMD for some of the stronger grimoires for either PVE or PVP and to get +3 AC from tumble, as the class also does not get it natively. I would consider this a budget Misfortune's Favor. You don't get +4 uni saves, you don't get defensive roll, but you get evasion, and if you take the Dark Blessing epic feat the uni saves wouldn't have mattered anyway.
You also need to consider +4 uni saves and defensive roll, vs access to Grimoire Timestop, Lesser Mind Blank, Breach. . .
Then you need to consider that in order to take advantage of its spell list, which you should since you can get pretty long lasting improved invis and extended haste, or other effects like Predator for blood count, you want at least 14 Charisma. A lot of your DCs also scale with charisma. I can see people going something like 16 / 8 / 12 / 14 / 8 / 16 and taking the +2 STR and CHA gifts, and then just going all in on STR from that point on, which means at best you're getting a +10 bonus from charisma modifiers to your ability DCs.
Assuming pure Harbringer, 12 + 15 + 10. That means most of your class abilities will have a dc of 37. Invocations will have a DC of 39. They can improved by +4 with Woe Drinker, which also increases damage. So, 41 for strikes and whispers, 43 for Invocations. If you take Hex of Acedia, 44 for Strikes and Whispers against targets of Dark Claim, 46 with invocations.
I actually don't think these DCs are bad at all. Combined with the fact that you can make Dark Claim also be a Breach 4, you could cast lesser breach, followed by this, and -maybe- you get to land your special effects. I can see a Harbringer actually managing to land its stuns and dazes if they go Dark Seraph in group pvp if there's a dirgesinger and someone with mindfog in their party. Not everyone is running 50 All Saves.
That said, assuming someone builds for the above, and they go all in on Harbinger:
I see divine patronage being taken 99% of the time. Since you're using charisma there's no reason to not go for a Divine Might / Divine Shield build.
I see Cursed Armaments being taken 99% of the time. Epic Prowess at epic levels is pretty good, uncapped Death Armor with refund is pretty good, and if you go for a 8 dex build you will need heavy armor. Access to exotic weapons without needing to take a feat is also pretty great.
And assuming someone goes for the above Hex of Acedia is the only fill slot. Where you may or may not take it based on wether or not you want to rely on DCs in PVP. I expect most people not taking it to favor instead something like Hex of Allucination since it helps with AB, or Mortality Undone for the DI and Epic Toughness, even if it ends up not matching what previous hexblade used to have.
If Cursed Armaments stacks with Eldritch Patronage I can see it being taken to maintain death armor, but I see this more as a PVE option. You won't run out of recasts in PVP before you've either died or the enemy ran out of non-item breaches. I also see it as low in the priority list to reapply if you've been hit by a breach. But, if you're taking this you're not taking Mortality Undone because of the ammount of class power slots you have pre-epic, so I also think even as a PVE option its a bad idea, as you're better off just not getting hit, and having death armor to do -some- damage in the fights you know you're going to get hit anyway.
Either way, I echo what Shadowy Reality said about the Improved Cursed Weapon line because of 5/- essences existing. Especially on Cursed Razor and Dark Seraph. Dark Seraph suffers from how popular entropy damage is at the moment, cursed razor is splitting damage between 2 damage types, which means potentially both can be hit with the 5/- immunity. Also, psychic damage is resisted by a surprising ammount of monsters in PVE fully. I helped a friend level a starpact warlock in the underdark recently, and until he could get his entropy beam (and withering ray) he was relegated to fire blades duty since most targets would only take magic damage from his eldritch blast.
Then there's the epic powers.
Woe Drinker on paper might be good for PVE and PVP for specific paths. I can see woe drinker permitting the Melee blood Count atrocity to actually be pretty disgusting as a finisher when people are low on HP.
The Breach Hex is insane.
The Oathbreaker hex is also insane.
Improved Dark Claim is bad. Its a purely PVE thing in my opinion, and goes directly against taking the breach or oathbreaker hex. Those two you want to use at the start of a fight, and they synergize with what the dark claims do. Count does a DoT, Dark Seraph increases its elemental damage and you want it to run out on something that's super tough when its low on HP so the DoT damage helps kill it. (I don't like this part of the Dark Seraph Dark Claim btw.) You also want cursed razor's hex to be applied at the start of a fight so you deal more damage with your atrocities and weapon imbue.
Crown of Malevolence is amazing since if you're going +10 Charisma you have little reason not to take it to get +10 uni saves.
Greater weapon hex is bad because you need improved weapon hex to get it, so it comes with the requirement of having given up a class feat slot for it. I, again, agree with alkmost everything shadowy reality said about it when he analysed this skill, I think in the context of what you need to give up for it, its even worse.
I don't like Misfortune's Favor, as if I'm going to give up an epic feat slot for evasion, I'll just take rogue levels.
Then there's the atrocities themselves and specific path abilities.
So far my thinking is that Blood Count(ess) and Dark Seraph are about on par with each other, and Cursed Razor needs a look at.
Even before I made the mistake of thinking Blood Count(ess) DR from killing a claimed target would stack with EDR for PVE facetanking, it seems like the comfy choice for PVE with its ability to self heal with almost all of its atrocities.
Its strike atrocity is probably the best out of all of them because of its scaling nature with missing % HP, and synergizes with its dark claim letting it heal and get DR from killing something with an atrocity, which is what you're going to want to be using this for in PVE. In PVP this ability is still a finisher, and depending on how it works, if its a free attack action like a divine smite, it can be used to snipe low HP targets in melee before they can use potions depending on how much damage you can frontload into it. It is also the only ability that has no saving throw to fully resist its unique effect.
I like it in concept a lot, I need to try it in practice.
Before I talk about Royal Decree I need to talk about all the ranged atrocities. All of them have relatively low the damage scaling. But at the same time, it is not written anywhere what type of action this is. If its a free action, its fine. If it takes an action to cast, it is very situational use.
The fear effect is meh. Again, depends on if its a free action or not so you can tell a pve target to go away for a while.
The kill on cast on 20% or lower hp target on a DC I'm not sure how I feel about. This is another ability that goes against Improved Dark Claim, since it already forces that roll if you actually took that ability, and if you don't, if the whisper is a free action and you're in melee with your hexed target I guess you could throw this out before trying to use the strike atrocity to guarantee the kill? Otherwise you can try to kill people running away from you on low HP, but the save being targeted is one that's commonly high, so I don't see this succeeding often.
Blood Homunculus access on a cooldown and vermillion stars is pretty cool.
The Invocation I'll need to test if it heals enough in PVE to be worth. Even if it was a standard action to use it might be good.
Cursed Razor I'm mixed on.
+4 DC on Bestow Curse. I only see this used for RP. Even with 100% refund on bestow curse, the best CC is either incapacitation or death. I do not like this.
Baleful Polymorph I like a lot more but I'm mixed on since I don't see the DC on it ever becoming viable since it doesn't receive the Atrocity DC treatment.
I don't see the greater curses being used in PVE either. Access to curse storm on a cooldown is pretty great.
The Harbinger's Curse worth will be judged by how impactful the greater curses can be in PVP. My only concern with this ability and the new curses in general is t he development of a meta where you hostile someone to curse them, then run away, and repeat until you've debilitated them enough to actually openly engage in pvp. This, of course, assuming they aren't a cleric with enough CL to just remove their own curses.
All of its atrocities have a DC attached to it that targets reflex to slow, immobilize, or force people near you. This is good against characters that dump Dex, but those are usually also characters you don't want to be close to, and might be able to outtrade you and kill you. More about this in Dark Seraph.
Dark Seraph I see being Cursed Razor but better.
It doesn't get any unique effects. Its elemental damage is just 1 type instead of being split to 2 so it can better bypass 5/- resistance and benefits more from the damage vulnerability (as you can roll more total damage).
Bleak Prophecy is entropy vulnerability. That's good. Bleak Prophecy's expiration effect goes against your goal of killing a claimed target before dark claim expires, and in PVP assumes that combat encounters are going to last longer than a minute.
Access to Black Blade of Disaster on a CD is great.
Hellstrike can be a 2 round stun. Slow is good. Stun is better. Stun flatfoots. Stun means no actions at all, not even running away. It is also targetting will, which I see as the better saving throw to target, as it is the most likely to not be that high especially on other martials. It will suck against clerics, and you can breach wizards. But all in all, if you're fighting a caster, you're usually winning if you're in melee and attacking them anyway.
Abyssal Lance is a daze. The teleport effect of Cursed Razor is probably better if someone is trying to run away from you, especially because the daze might only last 1 round, and dazed targets can still walk away. The teleport also can't just be ignored with a spell, while all of Dark Seraph's abilities are blocked by mind blank.
Apocalyptic cry I see mostly as a PVE tool that can make space. If all of these take actions to use, it is probably the weakest invocation out of the 3.
As for suggestions for improvement.
I'd raise the hitdice of the class. I don't understand why it should be a d8 hitdice class even with access to free epic toughness, especially with how competitive the class feat slots are when it is primarily a martial class like paladin and ranger.
I'd suggest revising hex weapon feats as because of the above I also don't see people taking them. If you're taking one, you're likely taking both, and I don't see it being that worth.
I'd consider maybe giving some more pre-epic class feat slots, if not just 1 more. 3 Feels really restrictive, as I see most builds taking the same 2 every time (I might be wrong).
Revise the effects of Cursed Razor's atrocities. Dark Seraph locks people down better if that was the intention. the - dex on the entangle isn't that amazing if the targets were already low dex. The people you'd want to lower the dex of are likely going to pass the reflex save. Both are equally affected by immunity to the effect they want to trigger. Dark Seraph hates mind blank, Cursed Razor hates Freedom of Movement. If this is meant to be the dedicated curse class, maybe make the atrocities actually interact with curses as a method of application if we're sticking with it being tied to a DC. Especially because Bestow Curse, even on a 100% refund, likely won't be used in PVE in favor of just killing things.
Thanks for clarifying. Looking good so far 
