Complex sentence: Due to John's disloyalty and usurpation, Alexios IV made his younger son Alexander of Trebizond "despotes" and married him to Maria Gattilusio, a daughter of Dorino of Lesbos.
Split into simpler sentences: Due to John's disloyalty and usurpation, Alexios IV made his younger son Alexander of Trebizond "despotes". Married Maria Gattilusio, a daughter of Dorino of Lesbos.