Battle Mage: A Hero's Welcome (Tales of Alus #8)

Sebastian had done the impossible. He had found the Lord Grimnal, who had disappeared generations before when the Dark One’s armies had trapped him on a distant island. His team of wizards and battle mages had driven off the wizard hunters and destroyed the barrier holding the legendary king.
Now all he had to do was bring him home.
The Dark One wasn’t ready to lose his prize, however, and he had other plans for his enemies. Using portal magic and the dark mage Palose, the emperor moves forward with the phase in his plan to undermine and destroy Southwall and its allies.
Sebastian and his team use their magic compass to discover the sudden surge in the amount of portals in Southwall, behind the great wall which has been the barrier to the emperor’s armies for over a century. In an effort to slow the emperor down, Sebastian and his team head to Litsarin to close the gates for good.
As the emperor moves his forces to counter each move of the battle mage, Sebastian finds himself in a fight against orcs, wizard hunters and the one known as the Betrayer.