Star Trek: Mirror Universe continuityEdit
Marcus' parentage was kept a guarded secret to avoid Kirk finding out about him. According to Spock, if Kirk had ever found out that David was his son, he would have killed both David and Carol.
In 2285, Carol recruited her son to assist her on the Memory Omega project. Together, they were the only two scientists on the project who knew the whole truth of it - that it was a secret stockpile of the scientific knowledge of the Terran Empire, to be preserved after its successor government, the Terran Republic, was conquered. David invented the quantum transceivers that made the project possible. For years, David lived with the other project workers, isolated inside the Genesis Cave of Regula I. (TOS - Mirror Universe novel: The Sorrows of Empire)
The Mirror Universe Saga continuityEdit
In an alternate continuity, Marcus led an underground resistance to overthrow the Terran Empire in the 2280s. Marlena Moreau was part of his group. When the Empire tried to invade the Federation Marcus' cell, along with members of the Federation USS Enterprise crew, freed some imprisoned scientists who helped them disable the invasion force. Marcus was able to capture many of the ships and emerged poised to overthrow the Empire. (TOS novel: The Mirror Universe Saga)