Ok so back in the day, we had The Snowmen and Series 7 part 2. Moffat is bigging up an old villain, The Great Intelligence, as the big bad of the series. Fantastically played by Richard E. Grant despite a vague motivation. I’m 15, and I’ve figured it out.
Missy isn’t on the scene yet. The last time we saw the Master is the End of Time where Rassilon’s plan was to make the Time Lords ascend to become beings of consciousness alone. What is the Great intelligence but a consciousness without a body? Hell bent on sabotaging the Doctor’s whole life after what? 3 failed attempts to take control. I thought I was onto Moffat. The Great Intelligence was the Master, free of their body, trying to get back to the mortal plane and wreak havoc. I like mine better