He was expecting a bit of resistance; being part Time Lord, River's obviously been taught a few things about mental shielding. He was not expecting that her mind has already been tampered with. The question is, by who? The Doctor? Or are there other Time Lords still lurking about?

"No more lies, River Song," the Master murmurs absently, concentrating mostly on pushing himself forward while guarding his own mind against any psychic backlash. He was always so much better at self-protection than the Doctor, but even without his own careful barriers in place, there are always the drums. He takes the essence of that beat, spirals it outward and into River's mind. The tendrils, hot, dry, baking with madness and rage, force themselves into the crevices, looking for weak spots. Cracks. Faults.

"Stop hiding, little girl."