Conjuration Ritual Spell

You have mastered Conjuration spells, what is next?

Walkthrough
Once you have reached 90 skill points in Conjuration, a new conversation option is revealed when speaking with Phinis Gestor. If you ask him


 * "What else is there to be learned about Conjuration magic?",

he will give you the spell Summon Unbound Dremora and tell you to obtain a Sigil Stone.

Summon and subdue an Unbound Dremora
To summon the dremora, you must first climb the steps to the roof of the Hall of Attainment. There, you will find a summoning circle (and on occasion, Nirya).

Enter the circle and cast Summon Unbound Dremora. Once the dremora appears, demand submission. The dremora will refuse and you will be forced to defeat it. Summon the dremora a second time and repeat the process. You may have to repeat the process several times, but eventually the dremora will submit, at which point you can instruct it to bring you a Sigil Stone. The dremora will then agree (grudgingly) and tell you that when you next summon him, he will have the Sigil Stone.

Deliver the stone to Phinis
Once you have the Sigil Stone, return to Phinis Gestor. He will then tell you that the stone is yours, and that he only needs to borrow it for a short time (seconds, really) to do something with it after which he gives it back. He will also give you the Spell Tome: Flame Thrall and thereafter, he will sell all the master level Conjuration spell tomes.