The Witness's presence inside the Traveler has manifested as psychic wounds, spreading physical corruption through its Pale Heart.
These concentrations of Darkness are dangerous. Ghost claims that the damage he took while fighting the Witness's possession is superficial, but you recognize when he's lying.
Both he and the Traveler are very hurt—and things are only going to get worse if you don't find a way to stop the Witness.
The Vanguard Warlock, Ikora, will know what to do. You must find her.