Find Kranal Fiss in the Barrens.
It's been some time since you were tested last, <name>. You've been patient thus far in your studies, but it's time for you to learn about greater powers... it's time you learn real power. The element of fire is yours 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 blasted night elf lands, you've gone way too far.
Hmmm, Swart sent you to me, did he?
Well then, Swart could always be trusted, so that must mean you're ready to take part of the next sapta--the sapta of fire.
I see it in your eyes already, the burning, the desire. You've tasted power, and now you're ready for more of it. Patience, <name>. Remember what earth taught you. Power and destruction will come easily, but you must still know when to stay your hand.