- In front of Zoro appears a skeleton with a large scythe (a shinigami).
So we fall into the same 3 cases:
1) It's simply Brook that Zoro mistakes for Shinigami because the medicine effect is so strong that is causing him hallucinations
2) The Shinigami is just a metaphor for Zoro's situation because he's currently struggling between life and death. So for example Shinigami will be represented like a giant shadow before Zoro, pretty much like Kaido saw 5 shadows behind Luffy in chapter 1001
3) It's an actual Shinigami
PS: if it's the 3) I'll put Shinigami on top of my Tier List, to be more specific in the God Tier which is even higher than PK Tier