I've cleaned the entire sphere out and I can't find the demon whos heart I need to power it to return. Does anyone know where this demon is and/or how to gain access to him? There is only 1 door I haven't opened and that's because it's a storeroom and the game tells me not to bother

Go to the sphere's exit (the same way you came in). Before doing that, its better to go to a room NE of the exit (its adjacent to the circular room with the 3 knights), to see what you will be up against :)

Thanks, didn't think of that