The old dragon that Roy mentioned was naturally the blue dragon Malygos, one of the Dragon Aspects of Azeroth.
The dragons, or rather, the Dragon Aspects, were a force that couldn't be ignored in the world of Azeroth. Many years ago, after the titans sealed the Old Gods of Azeroth, before they left, they bestowed powerful strength to the leaders of the five dragonflights and got them to protect this world. This was the origin of the Aspects.
These five Aspects were the red dragon Alexstrasza, the green dragon Ysera, the blue dragon Malygos, the bronze dragon Nozdormu, and the black dragon Neltharion.
And Malygos had been empowered by Norgannon, the Keeper of Celestial Magics and Lore and a member of the titan Pantheon. Thus, he was able to manage the magic energy of all of Azeroth. This powerful authority allowed him to obtain the title of Spellweaver.