In Welsh tradition, the area which became Brycheiniog was once part of the Roman territory of Demetia, to which was appointed a Greek named Antonius the Black (Anwn Ddu in Welsh) by Magnus Maximus, Roman governor of Brittania (and subsequent Emperor – hence, named Macsen Wledig in Welsh). Some Welsh legends describe Antonius as Maximus’ son, and name him the father of Saint Gwen, the triple-breasted (one of many saints he supposedly fathered). Following Antonius’ death, the realm was divided between his sons, Tudwal receiving the eastern portions, and Edynfed the western (part of which became Dyfed).

Historical manuscripts report that the next three generations of rulers of the eastern part – the descendants of Tudwal – were:
Teithrin ap Tudwal
Teithfallt ap Teithrin
Tewdrig ap Teithfallt
During this era, Irish raiders had begun to invade various western parts of Britain, with the Triffyn Farfog (son of Aed Brosc [it], a Deisi magnate) already having taken over Dyfed from Edynfed’s heirs. Traditional Welsh accounts claim that Triffyn had gained his power by peaceably marrying Edynfed’s heiress (they do not indicate whether this was under duress). The power of Irish raiders, particularly the Uí Liatháin, grew increasingly strong, and was not weakened until it was reduced by the sons of Cunedda, as reported in the Historia Brittonum.

Traditionally, Tewdrig carved out from his lands the region around Talgarth (Garth Madrun) as a dowry for his daughter, Marchell, when she married Brychan mac Anlach, the grandson of Triffyn’s fraternal nephew, Cormac mac Urb, in the mid 5th century.[2] The remainder of Tewdrig’s lands – Gwent – were inherited by Tewdrig’s son. Unfortunately, Brychan’s life is shrouded in legend,[2] but in any case, Brychan established a kingdom from the lands of Garth Madrun, which became known as Brycheiniog, after him.

– wikipedia

Brycheiniog is the kingdom in Lyonesse most directly threatened by the hordes of chaos to the north in Powys. Lacking the strong imperial presence of the kingdoms to the south (Gwent and Glywysing), Brycheiniog is a multicultral realm of free men, former imperial citizens, fair folk, and more than a few hobgoblins (the kingdom needs all the help it can get).

The survival of the kingdom depends on political alliance in the long term, but in the short term, heroes must rise to hold back the encroaching darkness long enough for that to happen. Needless to say, most of my default campaigns will be set in Brycheiniog. Now if I can just learn to pronounce it properly.

Here’s a map