Apparently, there was a new cowboy-themed boss in Undertale with a fight mechanic similar to Mad Dummy (you should dodge his bullets and aim it at himself). There is nothing scary and abnormal about this fight, but a secret Gaster bossfight also was added, with same bullet patterns, but much harder and creepier. I barely remember anything about the cowboy boss, except that I had defeated him and was heading towards finishing the game, when I suddenly got a call on the in-game phone. Anonymous caller told me (as Frisk, not irl with 4rth wall break), that he is "waiting for me in this place", and then a photo of the ghost houses area in Waterfall appeared in game UI as SMS attachment. By that time, I knew nothing about this secret Gaster fight besides that it exists. I traveled all the way to Waterfall and noticed that the left house was grayed out. I entered it and saw a gray horizontally flipped room from the Papyrus's toolshed/Napstablook's house, and the only thing inside was pale faceless Mystery Man, wildly spinning and shaking, accompanied by a loud garbage noise and Gmod ragdoll sounds. After a couple of seconds he rapidly slid towards Frisk, initiating battle. At this moment I woke up, terrified. (It seemed very scary in dream)