Hindu mythology had yogis. They were holy men who lived as hermits in the wild, spending their time meditating to gain spiritual and mental power. They underwent severe rites and penances to gain psionic powers. Mystic is an acceptable term for such a character type.
I'm not sure if they are Norse or Celtic, but another threat in the book was kelpies. They were malicious plants with the spirit of a siren who lured men to their deaths, the men would think they were embracing a beautiful woman but ended up drowning.
Some Forgotten Realms books reference Norse culture in the Ruathym and Luskan settings. The people from these nations are Illuskan, based on Vikings. They were predominantly pirates raiding islands and coastal areas. They were mostly berserkers, they had shamans who channelled ancestral...