I'm pretty sure this is Drarry but it could be Snarry.
All I can really remember about it is Harry and partner are captured and as they are about to escape the Death Eaters catch them. Next thing you know there is basically a Death Eater orgy and Lucuis ends up screwing Voldemort to death.
I'm not too sure about this but, I think Harry was given a fertility potion and one of them may have cast a spell or something to cause the orgy.
Thanks