An entity is merely any sort of spiritual being. It could be good or evil, but is most likely to be neutral. During the intermediate stage of Ki, you MIGHT get one of these. This is all of your bad energy within you escaping. This process makes you pure good. To make the room's energy good, do this technique:

Visualize an orb of dark energy in front of you. It is 1 meter X 1 meter in size and spins anti-clockwise. Now visualize the orb starting to slow down until it stops spinning. When it has stopped, visualize the orb turning to pure white from the core outwards. Next, visualize it exploding and white energy scattering across the room.

Demons are ancient creatures which have been training for eons. They are highly skilled but are not invincible. It's speed is so fast your physical eyes will not pick it up. But they can, and do, slow down to mess with peoples minds, bodies and spirits. You can sense it or see it with auric vision. They can open holes in time and space to travel from dimension to dimension. If you ever encounter one of these, I suggest you prey to your God if you are religious or try to be unimportant and it will hopefully go away without bothering you.