behold

This alerts the reader to pay attention to the surprising information that follows.

overshadowed them

"came over them"

Here "voice" refers to God speaking. AT: "God spoke to them from out of the cloud" (See: rc://en/ta/man/translate/figs-metonymy)

"the disciples heard God speak"

Here "fell on their face" here is an idiom. AT: "they fell forward, with their faces to the ground" (See: rc://en/ta/man/translate/figs-idiom)