Illusion Ritual Spell

The spell is a master-level spell-acquisition quest for the Illusion skill.

Walkthrough
It is obtained via a third conversation option from Drevis Neloren (located at the Hall of Countenance at the College of Winterhold) once level 100 skill level is achieved (see the bugs section below)

It involves finding four Master Illusion Texts scattered throughout the college that are only visible and obtainable through the use of a novice-level spell called Vision of the Tenth Eye that Drevis Neloren teaches you during the quest.

The books are located in the following areas:  (Don't forget to use the Vision of the Tenth Eye spell at each location!  If you tend to skip dialogue or skipped right to where the wiki states the locations, Drevis gives you the spell once you start the quest and it's located with all your other Illusion spells. Without using it at each location you won't be able to see the four books.)
 * 1) Hall of Attainment: Go up the stairs to the 2nd floor. It's under the bench on the left, at the landing.
 * 2) Hall of Countenance: Go upstairs to the 2nd floor. It's in the 1st room on your right, which is a storage room with crates and barrels. The book is on a barrel towards the back of the room.
 * 3) The Arcanaeum: Once inside the Arcanaeum, pass through the archway into the library proper. Before you get to the three steps down into the center, turn left into the outer ring. The first table with two chairs on the right hand side is where the book is. It can be on the floor in front of the bookcase too.
 * 4) The Midden. It will be in the middle of the table to the left, right before you reach the Atronach Forge.

When the books are returned to Drevis, the four books will be removed from your inventory and a master illusion spell tome will be added.

The quest-specific dialog options are available only when all 4 books are brought to him, and he says "Yes, very good. Here, then, is the first of the illusion spells you sought." Drevis never does give you the second, third, or fourth spells, but they instantly become available for purchase from him. The spells are as follows: Call to Arms, Harmony, Hysteria, and Mayhem.

Trivia

 * The books exist only when the spell Vision of the Tenth Eye is active. If one were to place an item where a book would be, the item will not be placed on top of the non-visible book, it will simply sit on the surface it was placed, and clip into the book when the spell is activated.

Bugs

 * It may happen that one of the Master Illusion Texts will not be in the location mentioned in the walkthrough above. This happens if the player accepted the quest Arniel's Endeavor while looking for the books.
 * The player may not be able to start the quest until after completing the Cleansing one also given by Drevis.
 * Due to a bug with Drevis' dialogue, the quest cannot be started until the Dragonborn has 100 skill in Illusion, despite the quest being able to be started at level 90.
 * Starting the quest, setting the Illusiion skill to Legendary and then collecting the texts may result in Drevis having no dialogue option to hand him the books, resulting in this quest remaining unfinished. A fix for this is to get one's Illusion skill back up to 90 or above, allowing the quest to continue. (confirmed on Xbox 360 and PC)

Der Illusionsritualzauber Ритуальное заклинание Иллюзии (Квест)