Zavala looks grim as he considers the possibilities behind a new Guardian-coded distress beacon sounding off from the Reef, especially given that it's rooted in an encoded Loyalist signature used by Calus. He laments that the Vanguard can't immediately respond to every call for help, but fears Caiatl is already on her way to this one and wants you to investigate it personally.
Zavala says your Ghost forwarded the signal to Rahool when you found it, and he confirmed that the transmission itself has been tainted by some sort of Darkness radiation.
He urges you to be cautious, and informs you that Osiris will run command on your mission. He suggests you don't go alone.