The slogan "Previously on Legacies" is heard at the beginning of all Legacies episodes since episode 2 (Some People Just Want To Watch The World Burn). These words are followed by a series of short clips that recap the scenes that are important for the current episode. All of this lasts for approximately one minute.
The purpose of these clips is to both remind viewers about current and past storylines, as well as aid comprehension for those who may not have seen the last few episodes of the show. In some cases, speech and actions are edited.
In every episode the voice on the "Previously..." is different. That's because different actors from the main cast are saying it. Normally the actor who does the voice-over for an episode is the character who is given the main focus of the episode, or is mostly centered on them.
Usually the actor that portrays a character that dies in the episode, does the voice-over.
List of cast that voice-over
So far in season one, Kaylee Bryant voice-overed 5 times, Aria Shahghasemi and Danielle Rose Russell 3 times, Jenny Boyd and Matt Davis 2 times each.