Was kind of expecting something... longer! XD He enters the room, drinks the potion, leaves the room, the end. It is good with a backstory, but without any surrounding intrigue it feels kind of... misplaced. For those who haven't seen the rest of these they have zero context too. Who's the blue demon? Why is this relevant? For us who have seen the rest it's relevant, but still a bit... explenationless. Did he plan to turn into a demon? Did he know what the potion would do? Did he change when he did take that potion? Questions amount!
Animation-wise it's good though, and the transformation flash in particular! Nice. Just wish there was more to it.
-cd-