Enma just depletes Zoro's Haki more than he intended, that's it !
Is it so hard to understand such a simple thing ?
:zorothink:
And yeah Enma bears not only Oden's will, who used it all his life, but also Kozaburo's will, a swordsmith AND swordsman who is the one who forged this katana (making him obviously the first to have ever used Enma).