Here’s a bit of a sneak peek towards what I’m thinking with the trust bites one shot for the lovesick AU and…
Owen roll up his sleeve, “I think…since we are bonded. You should have some of my blood. At least to make it official.
Legundo stared at Owen, yes, that would make sense… He was kind enough to allow Owen to drink some of his blood why wouldn’t his beloved return the favor with him even if he’s not a vampire. With gentle hands, he took his arm, treating it as if he was carrying a delicate flower. That same awful voice return albeit a lot more quieter screaming about this still being wrong.
“Please… you can’t have already forgotten.”
“Do no Harm!”
But what if someone end up hurting Owen. What’s the point of a vow like that if it made him miserable. Beside what’s the harm of loving someone?
Legundo then bites down on Owen’s wrist. After all these years, he thought he would fear it. After taking that pitiful vow he expected to fold, to pull once the taste hit him. Instead it’s like putting on a familiar coat. With each sip the bond between them grew stronger and the ever shrinking voice screamed that this is wrong faded away into nothingness leaving only his soon to be eternal vow echoing in his mind.
Something something, their bond being fueled by violence something equivalent in exchange
Unsurprisingly Owen is a bit more intense under the curse. Besides, I think the bitemarks are a good reference to how in many soulmates AU often times you get to know who your soulmate is through a mark! 
