There's already some anime where one girl transforms into the MC's sword, none have been yanderes that I know of.
Anyone remembers Seirei Tsukai no Blade Dance? I remember the MC has a yandere girl as his blade before getting the current one (in the show).
Now, let's fu**ing mu**er the demon lord and get married ♥No problem now, right?You can't cheat on me with other swords, okay?Daggers and knives are a no as well!Treasure me alone, forever and everThe price of such power was committing the remainder of my life.