During the fight with Clark, the Phantom Wraith takes some of Clark's DNA to become "evil Clark". This is Smallville's version of Bizarro, a result of a long ago Kryptonian experiment as explained by the Martian Manhunter. In the comics, one version of Bizarro was a laboratory-created evil Superman clone where another was an alternate reality Superman from a Bizarro World.
This is the 4th season which finishes with the credit "To be continued". Simultaneously, this is the 7th time which culminates with this expression. The others were: Tempest, Calling, Exile, Commencement, Oracle, Vessel and Salvation.