Complex sentence: Both Baha'al-Dawla and his brother, however, were threatened by their granduncle Fakhr al-Dawla, the ruler of Jibal, who invaded Khuzestan in an attempt to drive a wedge between the two brothers' territories.
Split into simpler sentences: Both Baha'al-Daula and his brother, however, were threatened by their granduncle Fakhr al-Daula, who was the ruler of Jibal. Fakr al-Daula invaded Khuzestan in an attempt to split the two brothers' territories.