Skyrim:Conjuration Ritual Spell

Quick Walkthrough
#Speak with Phinis Gestor, after reaching level 90 Conjuration, and receive the spell,  Summon Unbound Dremora.
#Go to the top of the Hall of Attainment and cast Summon Unbound Dremora. Speak to it, then defeat it.
#Cast Summon Unbound Dremora again and speak to it.
#Defeat the Unbound Dremora and summon it again. Talk to it and receive the Sigil Stone. 
#Talk to Phinis Gestor and receive your reward of the Master Conjuration Spell, Flame Thrall.

Detailed Walkthrough
To start this quest, go to the College of Winterhold when you have level 90 in Conjuration. Talk to Phinis Gestor and ask him if you can learn more about Conjuration. He will need a Sigil Stone. He will give you the spell Summon Unbound Dremora. Head to the top of the Hall of Attainment. Note that you need to go outside. Stand in the circle and use the spell. This will summon an Unbound Dremora. 
Summoning the Unbound Dremora will initiate a conversation:

"You dare bring me here? You will be punished.".

At this point, two dialogue options will appear:


thumb|The Unbound Dremora in combat|right
At this point it will engage in combat and you must kill him. After he is dead you must summon him in the circle again, and he will speak again:

"I am not your plaything, worm!"

At this point another two dialogue options will appear:



At this time he will become hostile and you must kill him. Summon him again and speak to him:

"I am not yours to toy with!"

From here you can reply with:



Both of these options will lead to you saying:

"Bring me a Sigil Stone. Now."

He will then say:

"Yes, my lord/mistress. Summon me again, and I shall have your stone."

At this point in the quest, he will banish himself and you must summon the Unbound Dremora once more. When you do it will say:

"Your Sigil Stone. Lord Dagon is... less than pleased at its loss."''

The Sigil Stone will be added to your inventory and the Quest Objective, Deliver the stone to Phinis, is assigned. Bring the Sigil Stone to Phinis Gestor and he will give you Flame Thrall. You will also, now have the option to buy the Master level Conjuration Spells; Frost Thrall, Storm Thrall, and Dead Thrall.

Notes
The Sigil Stone you are rewarded in this quest is used to unlock the full potential of the Atronach Forge. Daedric Atronach Forge recipes, both with and without the Sigil Stone, will begin to show up in random loot after you receive the objective to "Report to Savos Aren" during Good Intentions, regardless of this quest.
The final Unbound Dremora will remain until it is killed and can move outside of the circle in response to any threats or danger.
If you have the Disarming Bash perk, you can disarm the Unbound Dremora and then pick up his weapon.
If you cancel the dialogue and sneak behind it, it is possible to pickpocket its weapon with a high enough skill. It carries a random enchanted main weapon and an iron dagger. You may also attack it from behind to weaken it before talking to it and resuming dialogue. But if you kill it without resuming dialogue, you will not be able to summon it again.
It is possible to defeat the Unbound Dremora and pick up his weapon before he has been banished.
It is possible to get two Sigil Stones. Once the Sigil Stone has been received from the Unbound Dremora, it can be placed in the Atronach Forge. You can then go to Phinis Gestor and 'give' him the stone, when you talk to him again and ask "What did you see?" he will thank you and give you the second Sigil Stone and Flame Thrall.

Bugs
 
If it gets stuck, reloading and trying again should fix it.
 
Taking them to another zone and telling them to stop following you, will allow you to continue.
If you need to keep your follower, then summon something else far enough away (maybe a Dremora Lord if you purchased the spell) and tell your follower to attack it. While they are fighting, run to the summoning spot. The quest should proceed normally. 
 
 
If this occurs, loading the last save or autosave and trying again should fix the issue.
 
 
 
Completing that quest should fix the problem.
 
The quest starter dialogue option should still be available, going through the full dialogue again fixes the problem.
Reverse pick pocketing the sigil stone should allow him to recognize it in his inventory.
 
 
Raising your Conjuration skill to 90 or above once again will allow the quest to proceed normally.
 
Reloading the save and waiting until he is outside of the Hall of Countenance will fix this issue.

Quest Stages