Complex sentence: Gruffydd was recognised as his successor after an interview with Archbishop Hubert the justiciar, but Maelgwn used troops supplied by Gwenwynwyn ab Owain of Powys to attack Aberystwyth.
Split into simpler sentences: Gruffydd was recognised as his successor after an interview with Archbishop Hubert the justiciar. But Maelgwn used troops supplied by Gwenwynwyn ab Owain of Powys to attack Aberystwyth.