He could also be just an orlanthi with Darkness in him.
I'm playing a character in RQ with similar temperament and with the Air & Darkness runes as strongest runes. He's quite devoted to Orlanth, but is vengeful, likes to play it dirty etc. Good stuff.
I don't think cults like ZZ would be appropriate inside orlanthi clan.
Anyway, mortal people can and should be 'imperfect', and if someone goes against some of the cultural virtues, is a new / specialist cult is always needed? Iit's very 'Umath' to be unwanted and make a space for yourself, by force if needed.
One mythical option to explore: in Orlanth & Aroka myth Orlanth consorts with "Dark Woman" (Kyger Litor / Deloradella) who later gives bith to "Crushing Noise". Were he to approach Orlanth's Hall, and accepted inside, he would become part of Thunder Brothers. Ok, one without much Thunder, but hey, nobody's perfect.