Complex sentence: The priest decided to set up parish volunteers to keep vigil beside the image, and during the night, they did see the "urna" open by itself, as they saw with their own eyes the glorious image going out and coming back again.
Split into simpler sentences: The priest decided to set up parish volunteers to keep vigil beside the urn. During the night, they did see the urn open by itself, as they saw with their own eyes the glorious image going out and coming back again.