There is a famous saying in mathematics from Paul Erdős: "You don't have to believe in God, but you should believe in The Book." "The Book" is an imaginary book in which God had written down the best and most elegant proofs for mathematical theorems.
If there is a book written down by God, why not a book from the Devil? I mean, a book with the most ugly proofs, but yet the best ones we have as accepted proofs. I don't mean to make a horrible proof on purpose, but sometimes ugly proofs is all you have.
I wish if you could share some theorem from the Ugly Book, some theorem proved by a real ugly proof (and yet the only one that there is). I'm asking this not for fun only, but I'm curious about how ugly proofs can be.