The Heart Stone of Avooblis - The Adventurers' Academy, #6

Dagdron, Earl, Elloriana, and Lita's adventure has finally led them to Avooblis's homeland. With nothing but lava as far as the eye can see, the adventurers set off on the perilous journey in search of the Heart Stone of Avooblis. Desolate terrain and ferocious creatures await them around every turn, and they discover ominous memory portals defended by mysterious guardians. As the adventurers face the terrifying past of the land where Avooblis came to be, they must face an unknown traitor, an entrancing humming, and a rumbling volcano.
And on top of everything, each day that passes they're haunted by the realization that even if they fulfill their quest, they might never be able to make it back to their own land before Avooblis destroys it like he did his own.