What's unknown to rest of the islanders is that they secretly worship the forbidden deity Svalblod and all the blood they spill is meant to be in sacrifice to him. The remaining priests of Svalblod submit vildkaarls to trials meant to ensure only the strongest individuals pass and then recreate them into werebears ("berserkers" on Skellige) via ancient ritual.
In Skellige's formative years, when acolytes of Svalblod nearly drowned the Isles in blood, a short civil war erupted between the cultists and the rest. During it, berserkers committed mass murders and massacres until they were defeated. Worshipping Svalblod was then forbidden, his priests and followers bound on ships without sails and cast out to sea, henges brought down.
However, one small henge in Fornhala, the once-lively seat of Svalblod's cult full of ancient temples dedicated to him that had fallen into disrepair, survived. Fornhala's secret underground caves were where a few priests hid and began creating vildkaarls. Since then, warriors who have heard the "call of the bear" abandon their families and join the community.
They fought alongside regular Skelligers for decades and aside from building up a formidable myth-bordering reputation remained mysterious, revealing neither therianthropy nor cultism until 1272. After Konung Bran Tuirseach died, his wife Birna sought the vildkaarls out to recruit them for a coup which would turn Skellige into a hereditary monarchy with Svanrige Tuirseach as king. Their druid Artis was promised the position of advisor, hierophant title and lordship over Freyja's Temple with priestess expelled.