Vahan's army is going back with the Miners to the place where the mysterious stone fell.
Defeat more Skeletons. You must face their commander - Sialuv (1). He has a lot of health points, is resistance to magic, and can poison enemies. Avoid Sialuv after you start the mission - you will face him later.
Your army starts in the south (2), in the place where the boulder hit the ground. This group won't have any problems with dealing with slow Skeleton units. Sialuv is the problem here, but there is an easy way to deal with him. Simply attack him with Thurvid - you will slow the skeleton king down. Next, use the Miners and all ranged units (3). This method allows you to eliminate him long before other Skeletons can help their leader. Be sure that all your units are awaiting enemies in forests. Also, use the Miners' ability to defeat hostiles fast.