It was a prequel-era AU where one of the Jedi had had a vision of what Anakin would become, and the Council had ordered Obi-Wan to bring him in to be painlessly killed before that could happen. Cue much Obi-angst and a truly heartbreaking climax.
Anyone know what I'm talking about? I think it was on one of the bigger SW fanfic sites, and possibly one with an automated archive...