Sion

Creatures called Geists — malevolent entities that corrupt and consume people from the inside out — are winning a slow war against humanity. Shion is a warrior who hunts them down, and she's good at it. When she crosses paths with Hiroki, a young man who's been infected but hasn't turned, she does something unusual: she lets him live. His resistance to the corruption is rare enough that she'd rather fight alongside him than eliminate him. That uneasy alliance is where the story starts, and the two of them pushing into increasingly dangerous territory is what drives it forward. The dark fantasy setting has some genuine weight to it — the Geist threat feels oppressive, the power system where characters channel mystical energies has actual rules behind it, and the animation holds up well in the combat sequences. The soundtrack leans into the grim atmosphere without overdoing it. This is a four-episode OVA from 2008, adapted from a Lilith Mist visual novel, so it's short and doesn't overstay its welcome. Being a hentai title, it obviously carries explicit content alongside the action, so go in knowing what you're signing up for. If you've watched something like Bible Black or La Blue Girl and appreciated when that genre actually bothers building atmosphere around its story, this sits in similar territory. It's not reinventing anything, but the dark fantasy framing gives it more texture than most entries in the space.