Your Magical Name is Rina Witch

Mind-consuming creatures called Eaters have crept across the world so slowly that humanity's only real defense has become witches, and witches need mana. That's where Toshiya comes in. He gets paired with a witch-in-training named Rina at a school specifically designed to produce that mana through, well, intimate partnerships between students. It's essentially a magical girl setup where the power source is exactly what you'd expect from a hentai OVA. Don't let the fantasy framing fool you into thinking this is subtle about what it is. It's one episode, based on a visual novel from T-Rex, and it leans fully into the premise without much hesitation. What actually works here is that the setup is more thought-out than most entries in the genre: there's a genuine world with stakes, a threat that matters, and a dynamic between Toshiya and Rina that has some actual tension to it before things escalate. If you've seen something like Mahou Shoujo Ai or Discipline: The Record of a Crusade, you'll recognize the structure: fantasy or school setting, paired characters, adult content that's more connected to the story than it usually is in this genre. It's a single OVA, so it doesn't overstay its welcome. If you're looking for a hentai that at least bothered to build a coherent world around its content, this one delivers on that front reasonably well.