First, don’t forget that you don’t have to create the npc. They are represented as a difficulty. But what you do need is colour as to what they can do with their magic. I’d base it on a grimoire that they are associated with one or two runes, the rune(s) should give you an idea of capabilities and then wing it from there. You don’t need to know the name of the grimoire. If the npc is part of the group, remember the narrator shouldn’t be rolling against themselves, just say what would be good for the story.
Eg the group comes up against an evil sorcerer, dressed in black, with a tentacle (chaos and darkness). A shimmering blackness erupts from his fingers (He casts chaos magic on one of the group hoping to transform them), his level is let’s say difficult – and you’re off. He counters with a bolt of darkness, summons a broo, grows horns to gore someone, the earth beneath one of the party begins to boil, the sky darkens, etc.
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