DISCLAIMER WARNING: THIS THEORY IS NOT MINE
I saw this theory on the league of legends forums and immediately liked it. Although I have some other ideas about Zed, like the master is Zed, this idea is plausible. I don't know who thought of this, but I really like it.
zed became nocturne after being enveloped in the way of the shadow referencing to the quotes "What I have done... cannot be undone." and
"Do not fear the shrouded path." both suggesting that he cannot "turn away from the path he has chosen" and over time he slowly evolved into nocturne, and then nocturne came back from the future with the power of his futuristic skin "Eternum Nocturne".
As usual, tell me what you think!