Complex sentence: In the last year of his life he filled another prominent role, as member of a commission of prelates, magnates and justices appointed to hear the complaints of the "disinherited"-those who had sided with Simon de Montfort, 6th Earl of Leicester.
Split into simpler sentences: In the last year of his life, he appeared in another prominent role: that of member of a commission of prelates, magnates and justices appointed to hear the complaints of the "disinherited". That is to say, those who had sided with Simon de Montfort, 6th Earl of Leicester.