Carried by Savos Aren at the College of Winterhold. You can pickpocket it from him or [>>>Quest Spoiler Alert<<<] play through the College questline until the point where he is killed, and you can loot it from him.
Note that the game prevents you from looting Savos' corpse normally, and therefore you will have to use a mod such as Quick Loot or console it in.
Requires Royal Armory.
"Shalidor created this dagger early in his life and it was meant as a defensive weapon that takes an opponent's magical abilities and transfer it to the wielder. Other mages mockingly called the weapon The Hungry Knife."