Loved this chapter. I was really touched by how Kawamatsu and Onimaru saved each other from death, and I hope that they can rekindle that bond soon.
Oden's sword Enma should be exciting to see, but I don't think Zoro will leave Wano without Shusui.
As for Onimaru's ability, my personal theory is that he (the fox) ate the Hito Hito no Mi, Model: Benkei, a Devil Fruit based on the warrior monk Saito Musashibo Benkei. It seems more likely than a human eating a fox fruit. But, who knows? Maybe he's based off the kitsune from Yokai folklore.