As Regent
After Empress Jia, in jealousy, deposed the crown prince Sima Yu (born not of her but of her husband's concubine Consort Xie Jiu) in 299, there was a conspiracy to overthrow her and restore the crown prince. Sima Lun was persuaded to join the conspiracy, but Sun had another plan for him: instead encourage Empress Jia to assassinate the crown prince in exile, and then use the assassination as the excuse to overthrow her. Sima Lun accepted the plan and persuaded her to assassinate the crown prince, which she did in 300. He then declared a coup against her and arrested her, slaughtering her clan and her associates (including Zhang and Pei). He then forced her to commit suicide.
Sima Lun then became regent for the developmentally disabled Emperor Hui, but was described to be not particularly more intelligent than Emperor Hui. Even though he carried the regent title, true power was in Sun Xiu's hands. Under Sun Xiu's persuasion, he deposed Emperor Hui and declared himself emperor in 301, offering Emperor Hui the honorific title of retired emperor but putting him under house arrest. Emperor Hui's grandson, the crown prince Sima Zang (司馬臧), was executed.
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