What if the Reaper is Enma?
Enma is named after the King/Lord of Hell
Zoro just did something trying to pass Enma's test, by giving it all his haki without regulation
Maybe Enma does something like Samehada from Naruto, takes a bunch of Haki from the user, but after it gives it back to that the user can regenerate / heal
Samehada does it to enemies, but Enma could just do it to the user
So maybe we get Enma being like "Good, you pass my test, you are now my master, here have some haki back and heal, also i'm a black blade now because you completely mastered me to align me with your goals"