"Everything around me was eerily silent" Just like Luffy, this seems to translate to the state of near death.
"My heartbeat was like distant thunder" Obvious parallel to Luffy's drum like heartbeat when he awakened from his near death state.
"Standing at the gates of hades" Again, Obvious parallel, but this time it is in reference to Zoro becoming the king of hell which is exactly what hades is in greek mythology. Funnily enough Hades was aided by the dog cerberus, a dog with
THREE heads.
There are numerous routes Oda can take with this but its safe to say at this point that it'll lead to an awakening of some sort, whether that is a black blade or something more specific to Zoro as a fighter.