Yartek and some other Voord somehow managed to resist the power of the Conscience of Marinus. Due to this, they henceforth decided to invade the structure where the Conscience was kept. By retrieving the keys of Marinus and inserting them into the Conscience, the Voord would be able to control all of Marinus.
Inside the structure, the Voord killed Arbitan who was the keeper of the Conscience and Yartek dressed himself up in Arbitan's robe to fool the likes of the Doctor's party. But Ian Chesterton was not so easily fooled and he decided to give Yartek a fake version of one of the keys. With all keys in his possession, Yartek inserted the final fake one into the machine. As a result, the Conscience exploded, killing Yartek in the process.