Debris rains down onto the surface from the atmosphere above, left over from the destruction of Clovis Bray's space station, the Morning Star, after Guardians prevented House Salvation's efforts to use the Deep Stone Crypt to build an army of Darkness.
House Salvation now covets what remains, attempting to salvage anything they can from the debris themselves by fortifying the debris-laden areas through the use of Darkness.
"I believe House Salvation to be using whatever is at their disposal to secure and obtain the debris falling from the sky. Find them where the debris rains, and stop them." —The Exo Stranger