He negotiated an agreement with a Nimon that would give his people the ability to conquer the galaxy. Soldeed was utterly devoted to the Nimon. He believed wholeheartedly that it would fulfil its part of the agreement and would chastise anyone who questioned it. The Nimon trusted only him to enter its presence; anyone else would have been executed.
However, Soldeed was disappointed by the Nimon's refusal to advance him any further technology when the last shipment of sacrifices was lost. When the Fourth Doctor and Romana II appeared on the scene, he mistrusted them and tried to have them sentenced to death in the Nimon's complex. Afterwards, the discovery that the Nimon had been lying to him and intended to invade drove him raving mad.
Blaming the Doctor and Romana for interfering, Soldeed tried to kill them both and reverse everything the Doctor had done to prevent the Nimons from coming through their black hole. He was shot dead by Seth. (TV: The Horns of Nimon)