The inhabitants of Vinland will usually term any hostile use magic as "witchcraft". For game purposes, though, I use the term to refer to the particular practices among the Frankish immigrants around and to the north of Hvalrik (i.e. Boston). These came primarily in a large group around 1350, and their ships were the last contact with Europe that Vinland has seen.
Witchcraft here is a classic sort of European witchcraft. Its philosophy is a mix of Gallic pagan influences, Gnosticism, and some classical alchemy. At the heart of it, though, witchcraft is a craft: worked with the hands. The central principle of witchcraft is in shaping and using the magical power in natural things. Each living thing has certain magics within it. Minor uses of these are uses of medicinal herbs and so forth. More careful formulae will produce outright magic.
There are three primary skills of witchcraft: cauldron-craft, knife-craft, and hex-craft. Cauldron-craft works by distilling magical power via cooking and alchemical methods. Knife-craft works by carefully removing magical parts. Hex-craft works by internal mental methods.
Minor Cauldron-craft: Enhancing potions Fire-resisting ointment Love potions Poisons Poultices Sleep Dust Major Cauldron-craft: Cauldron of Resurrection Doppleganger Flying Ointment Homonculus Minor Knife-craft: Bane Arrow Blood Ring Necromancy Spankel Major Knife-craft: Transformation Items: Wolf's skin, Raven feather-cloak, ... Minor Hex-craft: Dowsing Empathic Cure Puppeteering Putrefecation Curse Visions Major Hex-craft: Evil Eye