Donald Morton was a man who had died and was brought to Carnon Manor.
Dr Morton rarely interacted with the other inhabitants of the house. He spent his time at the house looking for a cure for cancer, from which he had died about two months prior to the Fifth Doctor's arrival at Carnon Manor.
Because the house could only hold six people, and the Doctor and Nyssa's arrival brought the total to eight, the creature in the basement had to come for two of the inhabitants. It came for Dr Morton, because Lord Carnon found him useless. (PROSE: The Eternity Contract)