Honestly, I think people get tripped up on this question because 'The Only Blood' isn't like, one definitive title everyone knows? It sounds like a search term someone might use when they can't quite remember the actual book. If you're talking about the 'Blood Series' or something with 'Only Blood' in the tagline, the antagonist totally depends on which book. But if we're playing detective with the phrasing 'the only blood novel,' my gut says you might mean something like 'Only Blood' by some indie author, or maybe it's a fan nickname for a specific vampire book where blood is the sole focus.
I spent way too long last night down a rabbit hole on Goodreads and some old forum archives trying to pin this down. There's a novel just called 'Blood' by a bunch of different authors. Without the exact title, naming an antagonist is a shot in the dark. In one popular dark fantasy series I read, the 'only blood' theme referred to a prophecy, and the main villain was actually this corrupted former guardian who believed purging the world with a single, pure bloodline was the only way to save it. Twisted logic, but it made him a fascinating foe—not just evil for evil's sake, but convinced of his own righteous crusade.
If I had to guess the intent behind the question, it's probably someone who finished a book and can't remember the villain's name, or they heard a spoiler and are trying to context-match. My advice? Try to recall a cover detail or a side character's name. That'll get you closer than trying to describe the plot as 'the only blood novel.' The literary world's drowning in crimson titles, man.