Complex sentence: She met Jean Lambert Tallien, the Commissary of the National Convention, who saved her from the guillotine, and became his mistress-through her influence, Thérésa Tallien obtained the release of many prisoners.
Split into simpler sentences: She met Jean Lambert Tallien, the Commissary of the Convention, who saved her from the guillotine. She became his mistress and through her influence obtained the release of many prisoners.