*Willow possessed...FOUND!

[ladyhatshepsut.livejournal.com]

 The one I'm looking for takes place years after Willow did the spell to restore Angel's soul during the Acathla deal.  When she did that spell in her hospital room she was possessed by a demon or entity of some sort, which was the cause of her magic addiction.  Now, the others are beginning to realize this and work to contain her/its power and free Willow.  I think Ethan was tapped for help as the only one strong enough to keep her from lashing out.

*Found it!  Actually I stumbled over this storyline while re-reading 'It's Not Easy Being Green Carded' by gilesbabe.  Buffy and Giles are forced to get married when INS finds out that Giles, still a British citizen, has no job.  Toward the end of the story Chaos sends Ethan to S'dale, knowing he's the strongest person with no emotional ties to Willow.

gilesbabe has two great stories -- this one and 'Somewhere in Time', both Giles/Buffy -- at Just Rewards.  Check 'em out.