The Staff of Magnus is an ancient staff which is tied to the object known as the Eye of Magnus. Mirebelle Ervine states that according to legend the staff was handcrafted by Magnus himself, as it was the only object that could contain his power.
The Staff absorbs 20 points of magicka per second from the target. Once the target has no magicka left, health is absorbed instead. Magicka is absorbed into the Staff itself, so it is not technically a vampiric weapon.
The Staff is found in ancient Nordic ruin, Labyrinthian, it is the equipped weapon of the Dragon Priest known as Morokei.