what's the story behind the Arthurian/Coeling split?
Basically, many ruling Brythonic Llwyth/Lwyth(clan), be they the Northern Kings (obligatory "king in da north" line here), or the southern princes, trace their lineages to Coel Hen, a Romano-British leader who was the last Dux Britanniarum at the time of the Roman withdrawal, based in the Old North.
The exceptions being the brito-hibernian princes in Defyd, who trace back to the Irish king Anwn; The princes of Powys who trace theirs to Gwrtheyrn the usurper; And of course Arthurs' line, who trace theirs to Uthur Auriad "Pendraig", brother of Emrys Auriad (aka Ambrosius Aurelianus), they are allegedly the children of Imperator Magnus Maximus.
The Coelings generally view the Aurlings as usurpers, and say that if Uther really was a child of Maximus, that he was illegitimate. The Aurlings say that they are in fact legitimate descendents of Maximus, and that they have authority over all of Britain through him, especially because Maximus was the one who appointed Coel.
Suffice to say, the Coeling haven't liked any Aurling since Arthur.