The Seed was created during the Last Great Time War by the War Master as the ultimate warrior, his purpose being to infiltrate planets with a proven aptitude for war and to turn the populace into soldiers loyal to the Time Lords. He contained part of the Master's DNA.
During the Cold War, the Seed went to Earth and, as full-blown war did not break out, he failed in his task there. He was found in Arkansas in 1985 and sold to Richard Morris Temple. Missy, who called the Seed her son due to their shared DNA, would not allow him to be decommissioned and left him on another planet to rest. (AUDIO: War Seed)