Complex sentence: In the ceremony, he declared himself King Frederick of Jerusalem and immediately saw to it that his new father-in-law John of Brienne, the current Regent of Jerusalem, was dispossessed and his rights transferred to him.
Split into simpler sentences: In the ceremony, he declared himself King Frederick of Jerusalem. Immediately Frederick II saw to it that his new father-in-law John of Brienne, the current Regent of Jerusalem, was dispossessed and his rights transferred to him.