Shadow Mage: Blacklight Chronicles (Volume 3)

Talis glimpses an alien world where Rikar, his old friend and traitor, is being tortured. He is tempted down a dark path leading to the forbidden knowledge of Shadow Magic, the magic he needs to keep from going insane.
The feud between the royal houses of Naru has Talis and Mara caught in the middle, and once again threatens their friendship. And the magical Order of the Dawn distances themselves from Talis, pushing him aside to study the discarded magical art of runes. With civil war looming, Jiserian sorcerers again invade the city of Naru, and Talis must rise to help protect their city.
A mysterious sorcerer from the Tarasen Islands arrives, with eyes on the black crystal, aiming to create a World Portal to the planet where Aurellia and his Elders have escaped. Talis finds his heart wrenched between saving his city and saving the one closest to his heart. Even after being trapped on an alien world, with no way back home, Talis realizes that his friendship with Mara and Nikulo is the key to their fate and survival.