Alteration Ritual Spell

Walkthrough
Once reaching an Alteration level of 90, speaking with Tolfdir will reveal an option regarding learning more about Alteration magic. Select it, and Tolfdir will mention that he may be able to help if provided with dragon heartscales. However, the only weapon able to procure heart scales from a dragon is Kahvozein's Fang, prompting Tolfdir to ask the Dragonborn to retrieve the weapon.

Finding kahvozein's Fang
Due to Skyrim's radiant quest system, there are several locations in which the dagger may spawn. Volskygge, Ragnvald, High Gate Ruins and Forelhost are the most likely locations, but others may be possible.

Retrieving the Heartscales
Once Kahvozein's Fang is obtained, it must be equipped in the right hand (it doesn't work when equipped in the left hand), then examine a dragons corpse (it doesn't matter whether it's been recently slain or previously killed). This will eventually yield dragon heartscales.

It is possible to gather heartscales from a dragon that has been turned to ash.

It is not possible to gather heartscales from the skeletal dragon during the quest The Staff of Magnus.

Finishing Up
Bring the heartscales back to Tolfdir, and he will reward the Dragonborn's efforts with the spell tome Dragonhide, which allows one to ignore 80% of all physical damage for 30 seconds. In addition, the spell tome Mass Paralysis becomes available for purchase upon completion of the quest.

Bugs

 * Initiating the quest while Tolfdir is still standing outside Saarthal will cause the game to regard this as an ordinary quest, rather than as one related to the College of Winterhold. It will show up in the quest list with the default quest heading and won't count toward the number of College quests completed on the info screen.
 * It is possible to initiate the quest before starting Dragon Rising, but it would be impossible to complete it before Dragon Rising as no dragon corpses are available yet.
 * Sometimes the dialog option to give Tolfdir the heartscales will not show up. The cause is making Alteration legendary. To fix this, level up Alteration until it is level 90 again, then speak to Tolfdir.

Der Veränderungsritualzauber Ритуальное заклинание Изменения (Квест)