I found SNP's evil twin.
Ah, yes. You found them. They were a thing back in the 80s in the dark period under the SNP's leadership of Gordon Wilson (the worst leader the party's had), just like the left-wing, Republican "79 Group" which contained most of the Salmond cabinet (2007/2011 - 2014) among its members, including Salmond himself, as well as Kenny Macaskill, justice secretary of Scotland until a few weeks ago and so on. All the factions were expelled from the party by Mr. Wilson, who was a right-wing Christian Conservative Nationalist type, really into the oil and such. He later lost his seat, one of the safest the SNP had, but he deserved it.
Sìol nan Gàidheal were... hmm... "muscular" nationalists, as it were. Ethnic nationalists, not civic, we can say that much. They reveled in being accused of Fascism and all the rest of it and liked to talk about the English a lot. They were in their death throes in the '90s, but if I remember correctly they were pro-Serb in the Kosovo situation and moaned a lot about there being too many English people in the borders. They don't really exist now I don't think apart from an internet forum, a very-much "web 1.0" website and an overweight gentleman who likes to wear traditional kilts and get photographed by the Daily Telegraph. I actually met a few ex members on the referendum campaign and they were really nice people - not anti-English at all from what I could tell. I wonder what happened. One of them became a Conservative councilor, believe it or not. Pro-independence, of course.
Of course, Sìol nan Gàidheal were never much of a force at all but they were a gift to those who wished to destroy the SNP. Very useful in anti-SNP smear campaigns.