House Dondarrion
Marcher lords of Blackhaven under the purple lightning, born of a rider who outran two Dornish kings' men with a message and a storm at his back.
The fuller tale
The purple lightning on House Dondarrion's banner commemorates an ancestor who earned it the hard way — a herald riding a message north out of Dorne with the king's men in pursuit, who somehow outran two separate parties of them with a storm breaking at his back, a legend the house has worn on its shield ever since as though the weather itself had taken their side.
Blackhaven, their seat in the marcher country of the Stormlands, is held in this generation by Ser Manfred Dondarrion, whose family's connection to the tales runs through service rather than glory — Ser Arlan of Pennytree, Dunk's own master, once rode for Manfred's father, decades before the events the novellas actually cover.
That connection is worth exactly nothing when Dunk goes looking for it. Newly orphaned of his old knight and needing a lord to serve, he seeks Manfred out on the strength of that old loyalty and is turned away — the current lord does not remember Ser Arlan's service at all, and a hedge knight's decades on the road prove to carry no weight whatsoever with the son of the man he once served. It is a small, quiet humiliation, and it is what sends Dunk on toward Ashford instead.
The page and the screen
Whether this scene survives into the aired season in the same form has not been widely confirmed in reporting on the production; the encounter is a minor beat in the source novella and has drawn far less coverage than the trial of seven itself.