Find Kranal Fiss in the Barrens.
Long has it been since you were last tested, [player]. You have shown patience thus far in your studies, but the time has come for you to learn more. The time has come for you to learn about destruction... and chaos. The element of fire is yours for the understanding, if you're prepared to pay for it.
Seek out Kranal Fiss in the Barrens. He lives in a small dwelling north of the Crossroads, but be mindful, if you've reached the night elf lands, you've gone too far.