The Eye of Magnus is a mysterious object. Little is known of it, just that it is very powerful. The only known history of the Eye of Magnus is that it was discovered by the Atmoran settlers of Skyrim when they built their city of Saarthal in what is today known as the hold of Winterhold. When the Nords of Atmora discovered it, they believed it to be dangerous and too powerful to wield and so sealed it up in a secret area of Saarthal. However, the Snow Elves, or Falmer, heard of this and desired its power. And this led to the fabled "Night of Tears" where Falmer stormed the city of Saarthal and slew all men, women, and children in their way in order to stake claim to The Eye.
From here, we get the legend of Ysgrammor and his 500 Companions. He returned to Atmora with his sons and brought back 500 of the best warriors from Atmora to storm the cities of the Falmer. The Falmer were nearly driven to extinction from this reprisal and were forced to flee into the Dwemer cities where over the next several thousand years, they were poisoned by the Dwemer and reduced to the blind, animalistic creatures that the people of Tamriel today know and fear.
The Eye of Magnus made a second appearance in more recent days. The Eye of Magnus was once more discovered back in its hiding place in Saarthal and brought to the College of Winterhold for study. A Thalmor individual, present in the college as an advisor or spy attempted to take the power of The Eye for himself (or for the Thalmor, it is not really known). The current Arch-Mage, Artarya Indoril, of the College, however, managed to acquire The Staff of Magnus and stop the Thalmor individual and save the College and the rest of Winterhold. Possibly, all of Tamriel and Nirn itself.
While in the presence of the Eye of Magnus, a creature can elect to harness the power of Magnus through it. To do so, one must maintain concentration, as in concentrating on a spell. For each turn spent concentrating on the Eye of Magnus, a concentration save (DC 15) must be made. On a failure, Magnus' power is cut off from you and you take 1d12 psychic damage. On a success, you maintain concentration and must roll a Wisdom saving throw (DC 15). Failing the Wisdom save, you take 1d12 psychic damage. On a success, you take half as much damage.
While concentrating on the Eye of Magnus, Magnus's power allows you to cast spells without using your own spell slots. When casting a spell through Magnus' power, you gain 2d12 to all damage rolls made with Magnus' power, and any spell save DCs are increased by +3. Concentration spells cast through Magnus' power will immediately end your concentration on the Eye of Magnus.
If you possess and are attuned to the Staff of Magnus while attempting to harness Magnus' power through the Eye, the Staff will maintain concentration for you. The Staff of Magnus does not make any concentration saves and controls the power of the Eye perfectly. If not attuned, the staff will still handle concentration on the power for you, but you will be making the concentration saves. Any failed save will tear the staff from your possession, and the psychic damage you suffer is doubled.