The House of Mathrafal began as a cadet branch of the House of Dinefwr, taking their name from Mathrafal Castle, their principal seat and effective capital. Although their fortunes rose and fell over the generations, they are primarily remembered as kings of Powys in central Wales.
They—along with the Houses of Aberffraw, Dinefwr, and Seisyll—traced their descent from Merfyn the Oppressor who, along with his son Rhodri, established their control over northern and western Wales. Rhodri replaced King Cyngen in Powys after the latter died while on pilgrimage to Rome, allegedly because of his mother or wife (sources differ) Nest but more likely[1] through conquest. Cyngen's true heirs were either exiled or reduced to the level of minor land owners (e.g., the family of Sir Gruffudd Vychan).
Read more about the House of Mathrafal here: http://en.wikipedia.org/wiki/House_of_Mathrafal