Between the Elven scouts and the wyverns, they were building a detailed picture of the positions of the enemy.  But they didn't know why they were gathering and what this had to do with the warning from the Cult of Light.  So Anna decided that they should capture one or more Elves of the dark path.  If they were effected the same way as normal Elven folk by her aura created by her golden armour, they might learn something of the dark forces plans, they would target a spellcaster if they could.