Custennin is the first ‘tyrant’ mentioned by Gildas. He was a nephew of Arthur as indicated by a number of sources:
1. The Chronicle of John Hardyng:
“To Constantyne, duke Cader sonne on hye, / his neuewe was, for Cader was his brother, / As well was knowen they had but one mother.”
2. Brut y Brenhinedd p. 182v:
“… i nai ap i vrawd kanis mab oedd y kadwr hwnnw i wrlais iarll keirnyw o eigr verch amlawd wledig mam arthur.”
“… his nephew, his brother’s son, for that Cador was son to Gorlois Earl of Cornwall, by Igerne, Arthur’s mother, daughter of Prince Amlawd.”
as shown in the following extract from the manuscript:
3. ByA 32:
“Kustenin ap Kadwr ap Gwrlais iarll Kemyw nai ap brawd vnvam ac Arthur.”
It was because he was Arthur’s closest surviving relative that he inherited the crown of Dumnonia. His pedigree appears in Harl. 2414 and Llyfr Baglan. The former is shown below where the names appear as in the manuscript.
|Gen.||Harl. 2414b||Harl. 2414c||Harl. 2414d|
|-3||Pandwlff||Mor (m. Sglepiado[s])|
Note, Harl. 2414b is a continuation of Harl. 2414d. Harl. 2414c had been inserted in between as Gerdan and Owen were both offsprings of Maxen. Gerdan is probably Gratiana, daughter of Macsen Wledig, who according to Harl. 1974 30 and 31 married Tudwal ap Turmwr.
 Bartrum, P.C., 2009, 96.
 Grafton, R., Ellis, H., 1812, 146.
 Parry, J.J., 1937, 182b.
 Bartrum, P.C., 1966, 94.