The Dinefwr dynasty under King Hywel Dda would unite Dyfed and Seisyllwg into the Kingdom of Deheubarth in the early 10th century.

This war arose from the rival claims of Stephen of Blois and Empress Matilda to the throne England as descendants of William the Conqueror.

From that era, Lord Rhys would end up becoming one of the most successful and powerful Welsh leaders of the Middle Ages, fighting against King Henry Plantagenet as well as against his son, Richard the Lionheart, attacking his Norman lordship and capturing many of his castles.

[11] One of Rhys ap Gruffydd's descendant, Sir Rhys ap Thomas, would later be instrumental during the War of the Roses, being one of the leader and commander at the Battle of Bosworth Field, alongside the future Tudor King as well as Jasper Tudor, Duke of Bedford, and John de Vere, 13th Earl of Oxford.

He supported the next King Henry VIII, and joined him and Maximilian I, Holy Roman Emperor, of the House of Habsburg, at the Battle of the Spurs in France.

[15] The ancestral seat of the dynasty was Dinefwr Castle, initially built by the Prince of Wales, Lord Rhys, and eventually seized during the 16th century by the Tudor King Henry VIII, as the then owner of the House, Rhys ap Gruffydd, was accused of plotting with King James V of Scotland to replace him and become Prince of Wales.

He also married the daughter and heiress of Adam de Cardonnel, Secretary of John Churchill, Duke of Marlborough of Blenheim Palace.

His son David Reginald Rhys (1907-1991) married Anne Wellesley, 7th Duchess of Ciudad Rodrigo, a member of the Spanish nobility, and great-great-granddaughter of the 1st Duke of Wellington who defeated Napoleon at Waterloo.

She was also a first cousin of the 9th Duke who married Princess Antonia of Prussia, of the Imperial House of Hohenzollern of Hohenzollern Castle, and a first cousin of Lady Pamela Wellesley Grant, who married the grandson of Countess Hannah de Rothschild of Mentmore Towers, 3x great-grandson of Mayer Amschel, founder of the House of Rothschild.

[24][25] Their daughter Imogen Alice Rhys (1903-2001), married David Brand, 5th Viscount Hampden, Chairman of the English, Scottish & Australian Bank, and grandson of William Montagu Douglas Scott, 6th Duke of Buccleuch of Boughton House.