ANGUS ANGUS ANGUS!
A few things about securing your supercharge spell if you've stumbled here from a random Google search. You have to be at least level 35. You have to complete Vigrid Roughlands and Frostholm. You have to talk to Baldur Goldpaws in Olde Town to pick up the quest that sends you to talk to Erik Wyrndune. After that, it's just a matter of fighting a boss in Mirkholm Keep and finishing up the quest.
It can be a little confusing because it's not intuitive that you go talk to Baldur Goldpaws . . . nothing points you in his direction.
Best of luck!
So, beware crazy high health mobs of Dragonspyre! Angus is coming after you!