New main objective: Summon and subdue an Unbound Dremora
Note! This quest is optional and you will get the chance to approach it no sooner than after raising Conjuration to at least 90 points.
In order to activate this quest, you will need to find the mage Phinis Gestor and ask him if he can teach you anything more about Conjuration magic (screen above). Note - If you don't see this option, it might be a bug and in this case you will have to raise Conjuration to 100 points. Phinis will tell you of the Dremora and teach you the Summon Unbound Dremora spell, warning you that subduing the creature will be dangerous and require high skills. After you gain control over it, you will be able to order it to retrieve the Sigil Stone.
In accordance with the information received from Phinis, head to the Hall of Attainment and use the stairs to reach the very top of the tower. There you should find the area in which you are to summon the Dremora (screen above).
Press TAB, head to Magic, choose Conjuration and activate the Summon Unbound Dremora. Hold down the mouse button until the spell charges up (screen above).
If you do everything properly, the Unbound Dremora should appear. Speak with it and the Dremora will refuse to yield to you. As a result you will have to fight it (screen above), at best trying to keep it at distance. Luckily the Dremora isn't very resistant and you will need to keep attacking it until it disappears.
Once again activate the Summon Unbound Dremora and use it (screen above). Nothing will change overall, as the Dremora is still hostile towards you and you will need to defeat it again.
As you have probably guessed, you will need to Summon Unbound Dremora once again and speak with it. This time it should give up (screen above) and promise to obtain the Stone for you.
Wait for the Dremora to disappear and once again summon it. Talk with it to receive the Sigil Stone (screen above).
New main objective: Deliver the stone to Phinis
You can now return to Phinis Gestor and give him the Stone obtained by the Dremora (screen above). Wait a moment and speak to him again, asking what he saw (screen above). The mage will return you the Sigil Stone and additionally give you the Spell Book: Flame Thrall. You can also buy other books from him, should you want them: Spell Book: Dead Thrall, Spell Book: Storm Thrall and Spell Book: Frost Thrall.
The Sigil Stone that you got from Phinis (and previously from the Dremora) is quite important, as you can use it at The Atronach Forge found in the Midden (screen above) to create the most powerful and precious items.