Guesses

Underswap sans Finds a multiple au invitation in his mailbox, when he reads it he decides to go and see these People even though he didn't know what these "aus" were, but to join the party the invitation says to go through the lab. When blueberry does so, he trips over something making a glass box containing with a !?&@$!?$&@! shot fall on him and inject the fluids inside to him. Find out what will happen.