You get this codex automatically once you complete Flashpoint Depths of Manaan.
- "Even before the dismantling of Imperial Intelligence, no record existed of an SIS agent named Theron Shan. There are, however, vague accounts of a dangerous Republic asset dubbed “Technoplague” who may very well be Shan. The asset first came to be known through his role in the death of Darth Mekhis of the Dark Council. He was later believed to have surfaced during the Battle of Duro and to have been instrumental in the destruction of the Ascendant Spear and the death of Darth Karrid, Mekhis’s second successor to the Sphere of Technology. Karrid’s successor, Darth Acina, gave the asset the Technoplague codename, but whether the perpetrator of both of these events is the same, and whether said perpetrator is Agent Shan, remains to be seen."
- ―In-game Codex Empire Side (Persons of Note)