The twist: I bet Mordeskull corrupted Thresh, whom in turn set things up for the Ruination

Trapped in his evil skull, Mordekaiser must have been the agent turning Thresh into the sadist he is now, and set things in motion for the Isles' ruin, and his return, through Thresh.

The phrasing in the bio was vague, but was there ever a singular intent behind Thresh's corruption you bet it was Mordeskull.

It seems a bit too convenient that the Blessed Islanders wouldn't be competent enough to keep something like the Ruination from happening, just from someone uttering a dark spell, and even more convenient that the it blew up the vault Mordeskull was in! Surely there would have been magical wards, and safeguards against the dark things kept there from amplifying a spell like that? Someone must have saboteured the protective charms.

I call conspiracy!