Conjuration Ritual Spell

Upon reaching level 90, you can talk to Phinis Gestor to pick up this Quest. Phinis will teach you the Summon Unbound Dremora spell, which can only be cast on the roof of the Hall of Attainment (roof of your dorm; you can check your quest and set it as Active for a marker to appear on your map).

Once summoned, there will be some dialog and reguardless of your dialog choice the Unbound Daedra will attack you. A companion is helpful to tank damage, but be sure to keep your distance.

Cast Banish or Expel Daedra repeatedly to drain it's health

OR

Cast Destruction spells (preferablely not Fire as he resists it) and Summons.

Repeat 3 times and he will submit. Order him to retrieve the Sigil Stone. Then resummon him. Take the Sigil Stone back to Phinis Gestor (be sure to have this quest selected as the Active quest to use map to find him).

After the dialog you will get a free Flame Thrall Tome.

The Flame Thrall is a Master level spell and costs 534 magicka to cast without the Master Conjuration perk. It lasts indefinitely but still has HP. The Twin Souls perk works for them so you may also summon two of them (one at a time if you don't have enough magicka).

Phinis offers the other thrall spells for a considerable fee (roughly 3,000 Septims each).