Begin by identifying the components:
مُمَلَّكًا — an excepted king.
أَبُو أُمِّهِ — his mother's father, a maternal grandfather.
أَبُوهُ — his father.
These are familiar words. Yet assigning the pronouns to their proper antecedents and arranging the kinship relations in a transparent way requires more than an ordinary reading.
The defect lies not in the obscurity of the individual words but in the complexity of the structure that links them.