Complex sentence: The Lackfis and their immediate successors were the kings' loyal supporters, but Sigismund of Luxemburg appointed close allies of John Kanizsai, Archbishop of Esztergom, to the office, for helping him seize the throne in 1387.
Split into simpler sentences: The Lackfis and their immediate successors were the kings' loyal supporters. However, Sigismund of Luxemburg had to appoint the close allies of John Kanizsai, Archbishop of Esztergom, to the office, because they had helped him to seize the throne in 1387.