Arbitan

Arbitan was the Keeper of the Conscience of Marinus and father of Sabetha.

Biography
Arbitan guarded the Conscience from the Voords who were trying to capture it. However, he foresaw the need for the Conscience to be used and so sent several people out to fetch the four missing keys, including his daughter Sabetha. None of them returned and so, upon the arrival of the First Doctor and his companions, Arbitan blackmailed them into searching for its four missing keys. However, shortly after they left, he was stabbed and killed by a Voord who had been hidden nearby. (DW: "The Sea of Death")

Yartek, the leader of the Voords, pretended to be Arbitan by dressing in the Keeper's robe, keeping in the shadows and hiding his features beneath the hood of the garment. (DW: "The Keys of Marinus")