Eye of Magnus
The Eye of Magnus is a large, glowing spherical artifact that The College of Winterhold and the Dragonborn discover deep within the ruins of Saarthal. It is guarded by an equally mysterious and very unfriendly Draugr named Jyrik Gauldruson, who is initially rendered impervious to any sort of damage (physical or magical) as a result of the Eye.
After Gauldruson's defeat at the hands of Dragonborn during a College expedition into Saarthal, the Eye is transported to the Hall of Elements in the College itself for further study. The Eye plays a central role in the College questline.
The Eye is implied to be the reason that Skyrim's early Snow Elves attacked Saarthal in an event which will be known to later generations as the Night of Tears, as they coveted the power it potentially offers. Though the Nord king Ysgramor was first forced to flee Skyrim following the attack, he would eventually return with the legendary Five Hundred Companions to drive the Elves from Skyrim. He then ordered the Eye to be sealed deep in Saarthal afterward.
Indeed, anyone capable of tapping into the Eye is said to gain extraordinary magical abilities, including the invicibility first witnessed by the Dragonborn in his/her battle against Gauldruson when the Eye is first discovered. Only another relic called the Staff of Magnus has the ability to effectively control the Eye itself.