In Chapter 1038 Zoro wakes up and sees a hooded skeleton wielding a scythe ready to strike him while he himself can’t move at all.
I’ve seen some people throwing out that the reaper we see is the personification of Enma testing Zoro. I think it’s actually Sandai Kitetsu, and this will be the moment where Zoro first turns a blade black. I think each blade Zoro wields will turn black as he grows stronger, and it will be one sword at a time. I think right now Zoro is going to turn Sandai Kitetsu black first because of all his swords, that is the only one that is truly his.
Sandai Kitetsu is this legendary cursed sword said to bring death to all who wielded it, and Zoro seemingly was the first ever to tame this blade. There truly has never been a wielder for Sandai Kitesu, Zoro is its first. Enma had Oden and Wado Ichimonji had Kuina and a long line of Shimotsuki, Sandai Kitetsu is Zoro’s alone. It would be fitting for the first blade for Zoro to turn black, even if he doesn’t realize how he did it to be Sandai Kitetsu.
Also Sandai Kitetsu is the only one of Zoro’s swords associated with death itself, Enma is the king of hell but not really a reaper/killer. So if Enma had a personification it would be closer to an Oni or a Demon, where as Sandai Kitetsu would probably be represented by some type of reaper.
The meaning of “bringing death” would be so literal from Oda as death has quite literally come. Maybe all users of the sword who are close to death see this figure. Zoro in surviving would have finally conquered the death that was brought to all those who wielded Sandai Kitetsu. In turn getting the blade to become black.