Time Shall Not Mend by AJ Hall (aka ajhalluk.)
It's part of a series of HP slash stories (note - in this series, she has not been more explicit than a kiss between the established couple) featuring the unlikely pairing of Draco and Neville.
Even more unlikely is that this particular entry is a crossover with Lois McMaster Bujold's Vorkosiverse. Starring Ekaterin. And that, despite the desparate times and the fact that one is purely magic and the other is hard sf, it seems to be working thus far.
You absolutely have to know the LMB universe to enjoy this, but I don't think you need to be a slash fan at all.