Here's my list of previously written Halloween fic.
First up, while I didn't write it for Halloween specifically, is Shattered. Since it deals with Divia in the flashbacks, and finally reveals the reason for Methos' aversion to vampires, I think it's on the creepy side [g] This was, I think, the first thing I ever posted on LJ.
From 2006, Lavender at Midnight. Another Bloodties series story, and deals with Triona's belief that the ghost of Methos' long dead wife haunts the house they live in.
And from 2007, Silver Service. This was supposed to be a Secret Santa exchange fic for amonitrate until it went south. I think I may be the only person who has ever crossed Highlander and The Twilight Zone! I'm very proud [snerk]
Oh, and if you missed this years, Empyrean's Shores, well now's your chance [g] It's the only one that doesn't have Methos in it too. He's not very happy about that. Here's hoping he doesn't soap my windows or tip over the outhouse.