Oh, I know this one too. It's by ShadowsWeaver1 on the Spark archive.
The Assassin's Seduction
The Assassin’s Seduction by ShadowsWeaver1
Kagome searches for the missing link to the past she thought lost forever. Little does she know that the link she will find will bring her beyond any heights of passion she has ever known...all without ever leaving her room. Chatrooms can be very dangerous things, and very tempting. Will our miko fall to the temptation of the Assassin's seduction? Read and find out. Sess/Kags. One-shot. Hentai. Lemon. PWP.
Rating: NC-17 - Universe: Alternate - Status: Complete - Genre: Romance
Updated: 01-18-08 - Chapters: 2 - Reviews 157 - Words 18,731