I spent most of this week rereading an epic. I love long universes that I can sink into and really see evolve.
Of Old Mystics, by Wolfling and Magpie. It starts out post-Chosen when Giles, as head of the Council, rescues Ethan from the Initiative, which has been torturing him for years. There's h/c for some of it, and a more even relationship later as they learn to work together to fulfill a destiny they'd never known. Not your standard Giles/Ethan, but one that I keep coming back to, when I have the time of course. Really well written original characters.
In the words of the authors: "NC-17. Schmoop, angst, h/c, dark flashbacks, mysticism, fantasy elements, romance... You name it, it's in here somewhere. Slash - m/m explicit sex. Elements of kink. Drug references. Lots of magic."