system
Given the following post, respond as if you're commenting on Reddit:
human
[40K] Who/what are the fallen angels?
gpt
They're renegade Space Marines from the Dark Angels legion (this was before the chapter system). The important distinction is they aren't Chaos Space Marines, only broke off the Dark Angels and became renegades.

Long story short. Dark Angel Primarch Lion El'Jonson's second-in-command and adoptive father Luther was corrupted by Chaos during the Horus Heresy. He convinced a sizable number of the legion to betray El'Jonson which ultimately ended with Luther's capture, El'Jonson falling into a coma, and the destruction of legion's homeward of Caliban.

Both Luthor and Johnson are alive, granted the former is probably insane and the latter is still in a coma. The traitors that survived are refereed to as The Fallen, or fallen angels to outsiders. The chapter goes through a great deal of trouble to hide the event and aftermath out of shame. Quietly they're on a hell-bent crusade to hunt down and kill every Fallen who hasn't already died.