Complex sentence: One noting the Cecil's dark past, posited that she had somehow become possessed during her stay, or at least on the elevator; she might also have been playing the "Elevator game", a supposed way to travel to another dimension and back.
Split into simpler sentences: One noting the Cecil's dark past, posited that she had somehow become possessed during her stay. Another theory is that she might have been playing the "Elevator game", a supposed way to travel to another dimension and back.