Hywel probably already controlled Dyfed by the time he assumed his father's lands in Ceredigion. No king is recorded after the death of Llywarch in 904, and Hywel's marriage to Llywarch's only surviving heir probably ensured that the kingdom came into his hands. Hywel and Clydog seem to have ruled Seisyllwg together following their father's death and jointly submitted to Edward the Elder of England in 918. However, Clydog died in 920, evidently leaving the whole realm to Hywel. Hywel soon joined Seisyllwg and Dyfed into a single realm known as Deheubarth. This became the first significant event of his reign.