The Hrormir's Icestaff is a unique staff found in the Arcane University, headquarters of the Mages Guild. There is only one staff like this. This staff is also unique because it has a white icy skin versus the standard wood skin that most staves have.
This staff is a significant item in the Thieves Guild quest, "Misdirection." Hoping to commit a couple of high-profile thefts to remove Hieronymus Lex from the Waterfront District, Methredhel has the Hero sneak into the Arch-Mage's Quarters in the Arcane University in the Imperial City and steal it.
If the Hero is the Arch-Mage, they can simply walk in and take it. The staff will not appear until this quest is initiated and will remain inside the Arch-Mage's quarters, even after it's been given to Methredel.
The quest version of the staff is weightless, and thus has a different Item ID.
It is possible to return to the Arcane University after "Misdirection" and steal the Icestaff.