How to trigger quest: First, you must be a MASTER CONJURER to even get this quest to spawn. Once you have attained level 100 in Conjuration, you can go find Phinis Gestor in the College of Winterhold. Ask him if he can teach you "anything more" about Conjuration and go through the dialog to get this quest.
Note: HOW do you get to level 100 in Conjuration as fast as possible? Good question! What I did was get the spell " SOUL TRAP " and then I went to a GIANT CAMP . Go kill everything in the camp and than cast SOUL TRAP over and over on the dead body of a mammoth or a giant. That's all there is to it!
Wear regen MP gear and be sure to have the MAGE STONE RITUAL STONE and the " Well Rested " perk. Also, consider putting some perks into CONJURATION to get the " Half MP for APPRENTICE level spells " to make this MUCH easier.
Phinis will basically tell you that to gain more powerful Conjuration spells, you will need a SIGIL STONE from an oblivion gate. To get one, we must summon an UNBOUND DREMORA and make him do our bidding. He will than give you the spell " Summon Unbound Dremor a" and tell you to go to the top of the Tower of Attainment to do this task.
Head up to the top of the Tower of Attainment where you will see a circle. Now, use your new spell to summon a Dremora. The Dremora will be less than pleased, and after a short chat will attack. Go ahead and kill it. After you have slain it, summon it again. Pick either dialog option and then when it attacks again go ahead and kill it again. Are we having fun yet!? Summon the Dremora again and it will FINALLY submit. Now ask it to go and fetch you a sigil stone and it will tell you to summon him again and he will have it for you.
After it leaves, go ahead and summon the Dremora again and true to its word it will have the stone! After taking it, go find Phinis Gestor again and tell him you have the stone. He will borrow it for a bit. Ask him what he saw afterward and he'll keep it a secret, but will give you the stone back AND a MASTER level Conjuration Spell: FLAME THRALL , which lets you summon a Flame Atronach PERMANENTLY .
Note that the Flame Thrall spell is a freebie, but Phinis has three new spells to sell you: DEAD THRALL, FROST THRALL, and STORM THRALL , which like Flame Thrall all have permanent effects. You'll now be done with this quest.