The climax of a story is when the major conflict encounters a turning point after which the resolution is fixed. The climax may or may not be the most emotional point in a story, but it must be the point from which the end resolution is determined. The major conflict in The White Stag is that of attaining the promised land. Therefore the climax will be the moment at which the rising action and complications are blocked by an event or a decision etc. that results in the falling action leading to the resolution.
In The White Stag that moment comes when Bendeguz and Attila are confronted with the Carpathian Mountains and that seemingly impassable facade. The White Stag, the Hun's and Magyar's harbinger of success, then appears and leads the Hun-Magyar people through the Carpathian Mountains to the promised land on the western side.