It is revealed to the party that a crucial character isn’t supposed to exist. Through perhaps a curse, scientific experiments on the mother, or otherwise, the character wasn’t simply “an accident”, but supposedly impossible.
When the party becomes are of this, so does the universe, and it begins trying to “erase” the character. The character starts looking like a glitch in a video game. Eerily perfect squares and rectangles of neon colour and static appear and disappear at random, sometimes parts of them disappear. Even their voice begins to loop, stutter, or become overwhelmed by static. This gets worse every day. The party must stop it.