When approaching Sanyi (you don't have to speak to her, just walking near her is enough) she will request you to follow her through the mountains and start walking. You will encounter some monsters unusual for this area: Two level 22 模板:Mragga Spawn, which are easy enough to kill or avoid. From the exit of Copperhammer Mines she will head to the left down into a valley which is swarming with 石峰矮人. They won't attack her, but unfortunately she will stop there until you walk close to her again, running the risk of aggroing a horde of enemies. You have two options: clearing out the whole area of every single dwarf (requiring a quite strong party and careful maneuvering), or simply waiting a bit until the mobs have strayed far enough away from your aggro bubble to safely approach her.
Sanyi will resume her slow progress further to the left up into a narrow valley where she'll wait for you to speak to her again. After walking close to her again, Sanyi will start talking once more and finally transform into the demon 模板:Mragga (level 28 死靈法師). She will also summon three more 模板:Mragga Spawns to her aid. Compared to the hordes of enemies you have faced on your way here, this fight is ridiculously easy. Take her down first, then her allies, and return to Vanyi (who will have a perfectly plausible explanation for this) for your reward. Necromancers may want to use the 捕捉紋章 to acquire 古蘭斯的平衡 from Mragga.