Hildana Deathstealer is a level 80 rare val'kyr with 18.9k health. She can be found patrolling in two different locations in Icecrown. Those two locations are Jotunheim and Ymirheim, and I have indicated the areas in green on the map below. She is also one of three rare spawns in this zone. The others are Putridus the Ancient and High Thane Jorfus.
Hildana is part of the Northern Exposure and Frostbitten achievements. She drops Val'kyr Vestments which is a level 76 bind on equip mail chest piece with a random enchant. Like other Northrend rares, she also drops Abandoned Adventurer's Satchel, which generally contains about 20 gold, a full stack of frostweave cloth, and often times a few crystallized elements. She also yields a decent amount of experience for leveling characters.
She is very obvious when she is up because she is quite large and doesn't really blend into the areas that she patrols around at all.
Melee classes may have a little bit of trouble reaching her at first because she flies just ever so slightly and annoyingly out of range. If you have trouble hitting her just move backwards a bit until she starts following you and that should move her down enough to hit her.