What happened to Plerg was clear: a Venomshrike Horror destroyed the walls while the fangeds escaping from it attacked the city.
As for Shrosi, Sebastian couldn't figure out what happened there until he could see what the Venquilar contained.
At least, that was what he thought until he saw the skeleton. It was all clear. A huge breach, a stretch of land near it, a giant skeleton on the ground.
<Something similar to Plerg must have happened around the same time fell.>