Or maybe his "demise" was just him falling into bottom of the sea when his ship was brought down in flames as he faced off against his sworn enemy onboard (no actual death confirmation).
What actually happened was that he escaped into a coffin he brought along with himself after emerging triumphant from he encounter. And he took over this formidable foe's body to further lengthen his life span as well as strengthening himself.
But the body resided a strong soul, resisting the control, taking it over was proven to be difficult. Hence it would take many years before Darkwill finally had the body under his influence. But even then he wasn't able to escape from the prison he got himself in which ironically was his last escape resort when the ship went down.
And so his disappearance turned out to be many years to decades, long after people thought he was already gone. But he would appear and live once more, thanks to the once Naut's crew found something else more than just horrifying black oozes in the ocean.
It was actually Darkwill's prison they encountered.
Now he is currently somewhere, filling the years of void during the time he was gone, and patching himself up to the top condition, in order to rule over Noxus and push his plans into motions once more.
hint hint