Foror's Compendium of Dragon Slaying is a BOE epic mission book(Warrior and Paladin can receive missions), Which can be purchased at the auction house with WOW CLASSIC GOLD. You can click on right button to pick up the quest of Foror's Compendium of Dragon Slaying. Then bring it to Lorekeeper Lydros in the library of Dire Maul, and start to talk with Lorekeeper Lydros. The final quest will be The Forging of Quel'Serrar (Note: At this time, your package should have a purple prop A Dull and Flat Elven Blade). Now your task is to Onyxia's Lair to break into Onyxia and insert the prop into the ground to get the task. After handing in the quest, you will get the mission reward Quel'Serrar.