Had a person who wanted to be a knight of the cross in a game I was running. I let them do it based on backstory. The father used a blade, she trained growing up to wield it. And is passing it down to her.
Then added my own underlying twist, and the relation to this topic. You see her father was a cultist who used the black. The cult over the ages had gathered fragments from THE cat-o-nine tails used to give Jesus his lashings. Since Lucifer had tainted it, by compelling the guards to torture him, the sword forged from the Cat-o-nine tails was also dark.
The player attempted to be a knight of the cross with the dark sword, but little accidents/coincidences would corrupt her efforts. Later she found out about the sword, brought down the cultists one by one, including her father, and sought out a sympathetic Wizard (Harry) to help destroy the blade.
The real Knights get involved, and through her prior actions, and during a baptism of sorts, the blade is cleansed. The player enjoyed it, and that is the first rule of the game. HAVE FUN.
I believe there was an old Indiana Jones adventure based on the 4th nail. And how it relates to traveling gypsies.