So much like the deal to consent to The Monks’ assistance, Missy and The Doctor’s pact will probably lead to something much more sinister.
We certainly don’t know the true form of The Monks, and there’s a chance that their true form is a band of rogue Gallifreyans under the control of Missy.
Come to think of it, The Doctor’s offer to Missy for assistance during last week’s episode, “Extremis,” was pretty similar to the procedure that The Monks require to control the planet.
The ritual of The Monks requires consent via feelings of love, and as we’ve seen in their long history together, The Doctor loves The Master / Missy as a fellow Time Lord.