@xrogaan It's but a tiny part of a big army. The less people survive to join the main army, the better. The best case scenario would be none at all. No information can be more terrifying than some information and especially full information. In the end the big explosion would still be something the king of Ferme could accept and thus prepare for it. It's not like the 200 men were simply killed with a wave of a hand, which would be horrible news for him, suggesting someone possesses the power of a god. They were merely killed by a trap he can't replicate, but it's not otherwise incomprehensible as a weapon.
I'd expect Almus to imprison the surviving ones until the war is over. In practice it would be troublesome to guard a large force of capable fighters. Again, the less survivors, the better. If there are too many, he might just let them walk, with all the negative consequences.