Old Magic

Old magic is a "more basic" kind of magic than High Magic, requiring things like blood sacrafice.

Users
Unlike High Magic, members of Magical Guilds besides wizards have been shown not only having knowledge of it, but using it, so it can be assumed that any Weir or Faer can use it. And a kind of Old magic known as The Old Religion that predates wizardry, and there all magical guilds, the most logical explaination is that anyone can use it given proper instruction, or there is another magical guild out there, yet unknown.