Voldemort Feared Himself
A Boggart is a creature that transforms itself into the greatest fear of the person standing in front of it.
This was first introduced to us in the third book, Harry Potter and the Prisoner of Azkaban. Many fans wondered what shape the Boggart would take if Voldemort stood in front of it; they thought it would change into Harry Potter or Albus Dumbledore, the former being the only wizard to have defeated him and the latter being the only wizard that he feared.
However, Rowling later revealed that Voldemort’s Boggart would turn into a lifeless form of Voldemort himself.