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A vampire Batman AU where every vampire feeds on one particular person at a time, whom they “mark” as their “prey.” They can still bite and drink from other people, but it does not empower them as much as drinking from their chosen victim - for example, their regenerative abilities stop working if they are feeding on someone other than their prey, and the blood that comes from other people tastes different to them (not as good and addictive).
So Batman becomes a vampire and happens to bite the Riddler while on a mission. Therefore, the two form a weird bond, and now Batman is forced to drink only the Riddler’s blood in order to keep himself alive. The Riddler complies, seeing this as an opportunity to obtain a certain sense of domination over the Bat. Of course, things don’t go the way he plans, which results in a bunch of uncomfortable situations, bordering on mild riddlebat.