How to trigger quest: First, you must be a MASTER at ALTERATION MAGIC to even get this quest to spawn. Once you have attained level 90+ in Alteration (I BELIEVE it is level 90+, but I did this when I was at level 100), you can go find Tolfdir in the College of Winterhold. Ask him "what else" you can learn about Alteration magic and go through the dialog options that follow to start this quest.
HOW do you get to level 90+ in Alteration as fast as possible? Well, this one is fairly easy. First of all, after going through the College of Winterhold questline (specifically ’ Labyrinthian’ ) you SHOULD have the alteration spell " Detect Life ". If not; go check with Tolfdir and see if he sells it (buy it).
Worst case; you can do the side quest " Infiltration " and get it as a reward. Now, with that spell in hand, go to a busy city (like the Whiterun market place) and use it over and over. Presto! It takes some time, but it is EASY to do.
Once you’ve talked with Tolfdir you’ll realize that he is working on improving the EBONYFLESH spell by using dragon scales, but it seems he needs the real thing. Yikes! He will then tell you that there is a dagger known as " Kavohzein’s Fang " that is so sharp it can cut off individual scales! In order to help out Tolfdir, we need to find Kavohzein’s Fang.
Now, in MY game "Kavohzein’s Fang" was located in Forelhost, a dungeon to the south of Riften. I’m not quite sure if it’s in the same place in every game or not (let me know), but Forelhost is INTRICATELY CONNECTED to the side quest " Siege on the Dragon Cult ". You should REALLY just go do that side quest to get the dagger. Be ready, as you’ll be going up against a DRAGON PRIEST ! .
Once you DO have " Kavohzein’s Fang ", you now need to equip it and look at the inventory of a dead dragon. Of course, this means you may have to kill a dragon, but once you do you will automatically get the HEARTSCALES that Tolfdir needs. With them in hand, return to Tolfdir.
He will complete his experiment (that was fast!) and will reward you with the first ever copy of the Master Alteration spell DRAGONHIDE ! Note that Tolfdir also sells another master spell called MASS PARALYSIS . Both are awesome spells. And now that you are done helping Tolfdir this quest will be complete.