The Reachmen are a race, so that's that. But for a Nord to allign with their clans? That's certainly plausible. Make him someone who worships Daedra, like Hircine, and is sympathetic to the Reachman culture.
”The Reachmen, also known as the Witchmen of High Rock, are a tribal group of primarily Breton descent who inhabit the Reach in southwestern Skyrim and the neighboring Western Reach in the east of High Rock. Their ancestry is not solely Breton, but spreads across many of the known races of Tamriel. Although they share descent, Bretons do not consider themselves kin to the Reachmen, and the Reachmen do not see themselves as Bretons. Their culture is tribal and primitive; they mainly utilize primitive gear and hedge-magic, and typically inhabit makeshift fortresses built into caves and ruins throughout the Reach's battle-scarred landscape. They primarily worship Hircine, the most primal of the Daedric Princes, although some clans are said to also dedicate ceremonies to Molag Bal, Malacath, Mehrunes Dagon, and Namira. Their shamans venerate the Hagraven "matrons", who in turn grant them knowledge of nature magic.”
“After the Soulburst in 2E 579, many Reachmen aligned themselves with Molag Bal, and remained powerful enough to launch attacks on the Ebonheart Pact and Daggerfall Covenant's territories, even attacking as far afield as the Rift and Glenumbra. Concurrently, one clan of Reachmen known as the Winterborn fought a lengthy war with the Orcs for control of Wrothgar after King Kurog gro-Bagrakh reestablished Orsinium in the region. The war finally ended in 2E 583, when the last Winterborn warlord, Urfon Ice-Heart, was slain at Frostbreak Fortress.”
Long story short: The reachmen are comprised of many races (nords included over the course of time.) so definitely NOT nonsensical for a nord Necromancer to be a part of the reachmen and as it says above, many reachmen did dedicate ceremonies to molag bal (which in turn has a pretty good connection to necromancy counting the divine/daedric rivalry between Arkay and Molag Bal.)
"When the human race learns to read the language of symbolism, a great veil will fall from the eyes of men." ~Manly P. Hall
Most of my characters already worship Hircine, so that fits nicely.
I'm thinkin' Rageclaw Clan witch (stamcro) since they are a matriarchal clan and it's a female character, and I will very likely make her a werewolf, like most of my other characters.
The Reachmen are slightly different from all the playable races, but it seems highly likely their closest relatives would be both Bretons and Nords (and potentially Imperials given that the Longhouse Emperors, the last ones to rule before Varen, were Reachmen). And as I understand it they will accept individual outsiders who share their values into their clans, so it could be as simple as saying your character had a nord mother (or grandmother) who joined a Reach clan - bearing in mind people get most of their racial traits from their mother in TES lore.
Part of my warden's backstory is that she briefly travelled the Reach and lived with some of the Reachmen clans, getting to know them through nearby Wyrd Covens. Eventually she decided she wasn't willing to dedicate herself to Hircine (and certainly not to Molag Bal) and moved on, but she did learn to tame and ride sabre cats while she was there.
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