Vereth the Cunning on the Rise of Suffering wants you to kill Sapph, the Rider of Frost.
Sapph... Sapph's a death knight like no other. She'll shower you with sweet words and you'll swear that she has no evil within her heart, and then she'll run you through without even breaking her smile.
I have no fear that you'll be able to best Sapph, but do not grow over confident. Her strength lies in lulling her victims into a false ease, shortly after shattering them with a fierce barrage of frost attacks.
Do not let your guard down, and do not underestimate her.