Complex sentence: After their return, many Vlachs were unwilling to join the rebellion and as such Peter and Asen built a prayer house dedicated to Saint Demetrius won't help the Byzantines.
Split into simpler sentences: After their return, many Vlachs were unwilling to join the rebellion. In response, Peter and Asen built a prayer house dedicated to Saint Demetrius hoping to gain the Saint's favor against the Byzantines.