Tarlan saw Gulph fall from a burning tower during a final confrontation with their undead father. There's no way Gulph could have survived - even with his special abilities.
As for his sister, Elodie, Tarlans convinced that shes a traitor who betrayed the rebellion and her family just so she could have the throne to herself. With nothing left to believe in, Tarlan's prepared to abandon both the cause and his pack of wild animals, and head north.
But appearances can be deceiving. And in a world of magic and deceit, mistaking lies for truth can be deadly.
(From the Middle-School Fantasy shelf.)