Complex sentence: Ludger was successful in these things, and then returned to teach in the "Martinsstift", after which he and some others were sent north to destroy the pagan places of worship west of the Lauwers Zee.
Split into simpler sentences: Ludger was successful in his undertaking, and then returned to teach in the "Martinsstift". He and some others were next sent north to destroy the heathen places of worship west of the Lauwers Zee.