The Living Blade(Marvel, assassin)

The Living Blade is an assassin whom Black Widow encounters in Madripoor. He is the one opponent who strikes fear into her heart. He is referred to early on as "The Bogeyman," a moniker which Natasha considered a myth until she fought him herself. When Black Widow stands between The Living Blade and a group of women who have been kidnapped for sex work, The Living Blade walks away.