Talk to Zavala
Speak to Zavala in the Hall of Guardians at the Tower.
"Commander, we're getting a signal. Strong, stronger than any other Cabal signal we've heard before. It's coming from…Phobos. What's your order, sir? Should we scramble a Guardian?"
—Tower frame in the Hall of Guardians, just before the Phobos incident
2015.09.09 (The Taken King)