Very little is known about Elystan himself, but his descendants, including Cadwallon ap Madog, ruled a minor principality in mid Wales, part of Rhwng Gwy a Hafren - the land between the Wye and the Severn. An early Welsh genealogical tract links him to Gwrtheyrnion, while other descendants ruled Maelienydd and Elfael. Other descendants ruled Cedewain. These territories lay in an area roughly equivalent to the later counties of Radnorshire and southern Montgomeryshire, in today's county of Powys. It has been conjectured by P.M. Remfry that these territories formed a polity known as Cynllibiwg. However, this is not generally accepted.
A Welsh genealogical source makes Elystan the seventh in descent from Iorwerth Hirflawdd, who probably died in the mid 9th century.