The Losgadh

The Losgadh is a recorded mythical creature in Scottish lists without any detail and this tale redresses that. In the first days of Scotland being visited by Christian missionaries, some missionaries resorted to brutal tactics to persuade peaceful farmers and artisans to convert to Christianity. Uffa Atticus, the village blacksmith, led his villagers in acts of resistance. The missionaries used as, mercenaries, former Roman soldiers who settled and married locally to burn Uffa on a bed of straw and pitch.
Uffa cursed the missionaries as he was dying but after being saved from death by a magical local spring he undertakes a campaign to kill Christians and burn their churches. Uffa is a dark, burned-black monster with control over all forms of fire but also with the power to control minds when it suits him to do so, making him a formidable entity. Ultimately, Losgadh leaves Scotland and heads to London with the intention of killing the Archbishop of Canterbury and burning Westminster Abbey to the ground.
The story starts with Uffa's renaissance in the present time after a quiet period while he gathers supporters before a concentrated campaign against Scottish churches and religious leaders that leaves many worshippers roasted in their pews and pulpits. He heads south and creates havoc in Newcastle-on-Tyne and Carlisle before reaching London. The carnage is extreme in both places. At Carlisle, Uffa is joined by a couple of other presumed mythical beasts who share in the destruction and killing.
An investigator from The Aberdeen Mythology Institute who calls herself Threnody, catches the Losgadh's attention and fascinates him. Threnody is very white-skinned and Uffa in his matt-black burnt skin, recognises her as his opposite. They meet occasionally through the story. Threnody liaises with police to hunt Uffa down and goes to London to try and intercept and dissuade him from his intended act of violence - destroying the Archbishop and the Abbey.
The backstory of the magic pool in Uffa's village is important to the narrative. Threnody's backstory is also there and the final act of Uffa's destruction is conducted with the women who oversee that magic pool.