I've looked all over and can't seem to find this specific fic. I can't remember if it was a full story or just an Omake.
The start of it was during the fight between Keitaro/Motoko and Tsuruko (something about proving that K/M were a married couple, I haven't actually watched LH, just read some interesting fics...), and Keitaro finishes the fight by whipping out a Raguna Blade and holding it at Tsuruko's throat. It then went on about how Keitaro's mother was going to kick him out of the clan for using his magic, which she had apparently been trying to do for years. If memory serves, he had been taught by Xellos.
The start of it was during the fight between Keitaro/Motoko and Tsuruko (something about proving that K/M were a married couple, I haven't actually watched LH, just read some interesting fics...), and Keitaro finishes the fight by whipping out a Raguna Blade and holding it at Tsuruko's throat. It then went on about how Keitaro's mother was going to kick him out of the clan for using his magic, which she had apparently been trying to do for years. If memory serves, he had been taught by Xellos.