I think the last one is good too. If you refer to them as daemons in your book then I would keep that as well. Sounds great!

This one is the strongest of those, for me:

BLACK DAGGER BROTHERHOOD x RADIANCE

Ariadne and Azriel are being hunted by vengeful daemons, but their marriage has a bigger threat: his impure bloodline. If she learns who he really is, he’ll lose her trust forever - giving the daemons exactly what they want. #PitMad #A #F #R

The first two make me ask too many questions that I need to know to follow the story. But this one gets the story across much more without eliciting questions. I find myself not needing to know why the demons are vengeful to get the gist; you highlight the fact that this is a romance; and you frame the secret in a way where I know just enough about it to get why it could be a problem, both for their lives and their love. So this is a long way of saying that I think it's good. I don't recognize the comps but I admit I don't read much romance and also, I read less than an agent does, no doubt. If the comps are strong then you should be set with this. Perhaps another editor might comment and see it with new eyes, though :)

Ah! I rewrote them again with a couple other versions, too. Perhaps these are more clear? I kept daemons for now, but we'll see if that sticks...

When Azriel proposed to Ariadne, he swore to protect her from vengeful daemons, but after his bastard bloodline causes uncontrollable jealousy, he fears she’ll uncover his secret. If she does, she’ll leave and the daemons will kill them both. #PitMad #A # F #R

When Ariadne accepted Azriel’s hand, she had no idea the lengths he’d go to protect her from vengeful daemons, but after jealousy gets the better of him, she blames his bloodline. The truth about who he is could ruin their marriage and get them both killed. #PitMad #A #F #R

BLACK DAGGER BROTHERHOOD x RADIANCE

Ariadne and Azriel are being hunted by vengeful daemons, but their marriage has a bigger threat: his impure bloodline. If she learns who he really is, he’ll lose her trust forever - giving the daemons exactly what they want. #PitMad #A #F #R

Looking at your revised versions:

When Ariadne, a victim of the war, against daemons, marries Azriel, secrecy about his bloodlines prompts suspicion. <vague, these details seem unnecessary? If she learns the truth, she’ll leave him, but without his protection, the daemons will be able to finish what they started. <which was what? what's the truth? what's his protection? for me, this one was simply too vague

Ariadne and Azriel have one thing in common: they’re hunted by daemons and he has to keep her safe. <what are daemons? (we need to know this) why does he keep her safe and not vice versa? (we may not need to know this, but we'll be asking this if it's phrased this way) After marrying, Ariadne’s curiosity about his bloodlines grows. <what bloodline? why is she curious? He’ll do anything to keep his secret. If he fails, she’ll leave him and they’ll both be dead. <why? again, just too vague overall. The main conflict is obscured for me.

BLACK DAGGER BROTHERHOOD x RADIANCE

A loving marriage. A devastating family secret. A brutal fight against daemons. <laundry list, still don't know what daemons are If the truth is discovered, the marriage will end, and Ariadne and Azriel will be killed. <why and why? Can he keep her from learning the truth and save them both? <still too vague.

IN CONCLUSION

Vagueness is the issue for me. What are daemons? Demons? (You can misspell it from book-specific language if there's another word that anyone would easily recognize. I've just seen daemons mean other things than demons.) Must the demons even be included? Are the two characters demigods or humans or...? What prompts the suspicion and why would the secret be deadly? (Those are the main questions to answer.) One solution could be to frame the story through one character, say Azriel. Eg:

When demigod Azriel married a human, he swore to protect her--but his bloodline isn't as pure as he's told her. When [inciting incident] happens, he must [do what?]--or else his wife will leave him. If both of them don't get killed first.

I like the original second as well but I wouldn’t reveal that the marriage will end if she finds out the secret. I would edit the verbiage in a way that says it would or could threaten the marriage, crumble it, threaten the integrity... something that keeps the reader guessing the possibilities.

I'm definitely still partial to the second one but the others read a bit clearer for me now!

Alright! I tweaked them all a bit... how do these sound?

When Ariadne, a victim of the war against daemons, marries Azriel, secrecy about his bloodlines prompts suspicion. If she learns the truth, she’ll leave him, but without his protection, the daemons will be able to finish what they started. #PitMad #A #R

Ariadne and Azriel have one thing in common: they’re hunted by daemons and he has to keep her safe. After marrying, Ariadne’s curiosity about his bloodlines grows. He’ll do anything to keep his secret. If he fails, she’ll leave him and they’ll both be dead. #PitMad #A #R

BLACK DAGGER BROTHERHOOD x RADIANCE

A loving marriage. A devastating family secret. A brutal fight against daemons. If the truth is discovered, the marriage will end, and Ariadne and Azriel will be killed. Can he keep her from learning the truth and save them both? #PitMad #A #R

Love the intrigue!

Here's my take on the plot twist element. You definitely want the twist to be kept secret from readers, but these pitches are for catching the interest of an agent. So I think you'd be ok sharing more of the twist if you chose too...but that's just my take.

My question on these pitches is: is the marriage the only conflict? Are they fighting together against something outside or is the drama between the two of them the only source of conflict? In the first pitch you hint that the daemons are still after Ariadne...how does that fit into the story?

My guess is that Azriel is a daemon himself but he loves Ariadne and he's trying to protect her from the daemons that want to kill/kidnap/harm her. He fears Ariadne will leave him if she finds out the truth. If that's right, you have two conflicts to work into the pitch, the outside threat and the threat to the marriage. Right now, with the focus on the marriage issue, the pitches don't have quite enough punch. Can you work the outside threat in to up the stakes in the pitches?

Sounds really good! I like the second one the most. Because the idea of the marriage ending over the secret feels like it comes from Azriel's interpretation of those consequences. Like, he is assuming she'll leave, which could be wrong but he's going forward on that assumption anyway. And I'm guessing the secret of Azriel's bloodline is connected to the antagonists of the war? If so and there's a way to work that in I'd add it.