Very specific materials, as well as extremely difficult “training” for the sword. Remember, enchanted weapons “learn” their specific enchantments through repeated use. Artificers can help move things like that along, co-opt parts from previously used weapons and whatnot, but on a new weapon, it’ll still take some personal experience to get the effect you want out of it.
I assume it’s so rare because it costs insane amounts of XP?
Very specific materials, as well as extremely difficult “training” for the sword. Remember, enchanted weapons “learn” their specific enchantments through repeated use. Artificers can help move things like that along, co-opt parts from previously used weapons and whatnot, but on a new weapon, it’ll still take some personal experience to get the effect you want out of it.