When this blob of night approached the Dragonlites, they realised it was just a bunch of crows. They did not take them seriously, opting to wait for them to get close enough to get turned into roasted chicken...erm bird.
Many in the frontlines laughed when those suicidal crows voluntarily splat themselves onto their shields as they instantly evaporated from the heat. They had mistakenly believed that their enemy was resorting to such a tactic to waste their mana so that they would have less for the later fights.
Because of this lapse in judgement, some of the mages slacked, and part of the shield became less reinforced, allowing the crows to penetrate through the shield. After the sacrifice of their predecessors and the cackling crows zoomed through and caused significant damage to the frontlines.