The Great Intelligence; also seen before in Classic Who.
Notice how it always takes a different form. In Classic Who it was ”a mass of tentacles and mouths” but took over other life forms too. In The Snowmen it’s snow and in the episode The Bells of Saint John it’s Wi-Fi (with an actual proper face this time). Also notice how in the last two episodes it took over kids, first Walter Simeon, who was just a little boy when the snow started talking to him. And second Miss Kizlet, who still had the mind of a little girl when the Great Intelligence let her go.