It appears this was basically the personal grimoire of black mage, and it was buried with him and has long since rotted.
So you have a couple of methods to get the information inside it, necromancy to ask Gottskalk himself, and a lot of research into whatever techniques he would have learned to contribute to his grimoire.
Based on the description, that the book contained a mix of runes and Latin, my guess is he probably created his own rituals based on a mix of runic and xtian magick, probably inverted. My feeling is the power came mainly from the runes, and some from the strength of his faith in xtianity as a current with power.
There’s not a lot online to find easily, they’re all regurgitations of each other and don’t say anything new, and I’m not sure I trust any of them really, or rpg game references.
This ref has a quote from a “folklore story retold by Shaun D. L. Brassfield-Thorpe”, so if you can find that original story, it might have more info.