Enma is an O Wazamono that was once wielded by Kozuki Oden along with his other sword, Ame no Habakiri. After Oden's death, it was bequeathed to his daughter, Kozuki Hiyori, who passed it on to Roronoa Zoro.
It is also, along with Ame no Habakiri, the only known weapon that has been capable of wounding Kaido.