Complex sentence: Eventually, Satan does discover the true circumstances of Edith's death (murder; that revelation and the motive for her murder did not make her very happy), and in the final episode Thomas discovers a fact about his ancestry that makes him even more insufferable.
Split into simpler sentences: Eventually, Satan does discover the true circumstances of Edith's death (she had been murdered: but that revelation, and the discovery of the motive for her murder, still did not make her very happy). In the final episode, Thomas discovers a fact about his ancestry that makes him even more insufferable.