Arniel's Shade is a ghost of the College of Winterhold mage Arniel Gane, who was destroyed or possibly transported to another plane at the conclusion of the quest "Arniel's Endeavor." The Dragonborn will witness this unexpected occurrence, when given the spell to summon the apparition, which is called Summon Arniel's Shade.
The spirit appears just as Gane himself did the last time the Dragonborn saw him in person, except that the robes he's wearing include a hood. He is summoned by a two-handed Conjuration spell that requires no Magicka, and he remains with the Dragonborn as an ally for sixty seconds. The ghost will engage any nearby enemies (including dragons), and he moves aggressively around the combat area attacking with a continuous lightning stream that appears to be a stronger version of the Sparks spell: it may be true that this is the only method of attack he makes use of.
Arniel's Shade, from the time he is summoned to the time he vanishes, certainly appears to be beyond injury of any kind, and elsewhere this has been attributed to his ghostly nature. It's possible that he has never been observed on his hands and knees in a nearly incapacitated state, as has been the case with numerous other followers over time; furthermore, it is also possible that he has never been dispatched by an opponent before the spell's sixty seconds of time has expired. However, when not in combat and following the Dragonborn Arniel's Shade can be heard gasping or perhaps retching from time to time, as if in pain or suffering.
The Dragonborn, if close enough, is given the prompt to talk to the spirit when not in combat, but he does not respond to this and no dialogue options are available. Consequently, just like a summoned Atronach but unlike a genuine follower, Arniel's Shade can't really be interacted with, insofar as he cannot be asked to wait someplace, or carry some of the Dragonborn's extra weapons or gear.
However, while the Dragonborn can't interact with the ghost, the ghost does attempt to interact with the Dragonborn. If his sixty seconds of time are nearly gone and he is not in combat, he will periodically speak to the Dragonborn when nearby: sometimes it's nothing more than the general question, "Yes?" At other times the ghost seems to actually recognize the Dragonborn personally, and, depending on the circumstances, will say something specific, "What can I do for you, Arch-Mage?"
Arniel's Shade is a shrewd, powerful, evidently invincible temporary follower that the Dragonborn can place on the battlefield in moments.