Season one sees the introduction of the regular cast, and storylines which regularly included a villain deriving a power from kryptonite exposure; the one-episode villains were a plot device developed by Gough and Millar. The first season primarily dealt with Clark trying to come to terms with his alien origins, and the revelation that his arrival on Earth was connected to the deaths of Lana's parents. After the first season, the series used fewer villain-of-the-week episodes, focusing more on story arcs which affected each character and explore Clark's origins. Main story arcs include: Clark's discovery of his Kryptonian heritage; the disembodied voice of Clark's biological father Jor-El is introduced, communicating to Clark via his spaceship, setting the stage for plots involving the fulfillment of Clark's earthly destiny; Clark seeking out three Kryptonian stones, at the instruction of Jor-El, which contain the knowledge of the universe and form his Fortress of Solitude; Clark battling Brainiac in his attempts to release the Kryptonian criminal General Zod; other escaped Phantom Zone criminals Clark must either capture or destroy; the arrival of Clark's biological cousin Kara; and Lex finally discovering Clark's secret. The eighth season features storylines involving the introduction of Davis Bloome, who is Smallville's interpretation of Doomsday, and a woman named Tess Mercer to replace the exit of Lex Luthor from the series. Justin Hartley reprises his role as Oliver Queen/Green Arrow, being upgraded to a series regular. With the ninth season, "Major" Zod (Callum Blue), along with other members of Zod's military group, are revived by Tess Mercer, though without their Kryptonian powers. Their efforts to obtain those powers become the central conflict for the season's story arc. The tenth and final season revolves around Clark's attempts to rid of his doubts and fears in order to become the hero he is meant to be while also confronting his biggest challenge in the coming of Darkseid.