Complex sentence: At the age of ten, he was engaged to Margaret, daughter of Floris V, Count of Holland, and an opulent psalter was being prepared for the marriage when he died a few months before the wedding was to take place.
Split into simpler sentences: At the age of ten, he was engaged to Margaret, daughter of Floris V, Count of Holland. An opulent psalter was being prepared for the marriage when he died, a few months before the wedding was to take place.