Granted. The sword you have wished for is, practically useless in every way imaginable for most normal swords. It is made of plastic, and has a poison coating, in which the poison only affects you, and you only in the world. Slowly killing you over the course of a 5 hour period. The pain you feel starts off as that of having an arm sawed off, and will continue to amplify 3 fold every hour. Due to this, the sword, will, by some fate, kill black as light as well. but it is swift and painless (they will be revived shortly after, as your wish doesn't state anything to them being allowed a perma death.)
I wish for a fluffy hat.