The regular cast is introduced in season one, with storylines involving a villain deriving power from kryptonite exposure. The one-episode villains were a plot device developed by Gough and Millar. Smallville's first season primarily dealt with Clark Kent's coming to terms with his alien origin and the revelation that his arrival on Earth was connected to the death of Lana Lang's parents. After the first season the series had fewer villain-of-the-week episodes, focusing instead on individual-character story arcs and exploring Clark's origins. Major storylines include Clark's discovery of his Kryptonian heritage and Lex Luthor's escalating conflict with his father, Lionel. The disembodied voice of Clark's father, Jor-El, is introduced; he communicates to Clark through his spaceship, setting the stage for plots involving his role in fulfilling Clark's earthly destiny. In a fourth-season arc Clark, instructed by Jor-El, searches for three Kryptonian stones which contain the knowledge of the universe and form his Fortress of Solitude. Clark battles Brainiac in his attempts to release the Kryptonian criminal General Zod, and must capture (or destroy) other escaped Phantom Zone criminals. His cousin Kara arrives, and Lex Luthor discovers Clark's secret. The eighth season introduces Davis Bloome (Smallville's version of Doomsday), and Tess Mercer replaces the departing Lex Luthor. Justin Hartley becomes a series regular as Oliver Queen (Green Arrow) after being a recurring guest in season six. In the ninth season Major Zod (Callum Blue) and other members of Zod's military group are revived (without their Kryptonian powers) by Tess Mercer, and their efforts to regain their powers are the season's central conflict. The final season revolves around Clark's attempts to lose his doubts and fears and become the hero he is meant to be, while confronting his biggest challenges: the coming of Darkseid and the return of Lex Luthor.