Source (Tumblr), alternate frontend.
I can do one better, in a campaign I was in a few years back it turned out that due to some time travel shenanigans and an attempt to find a loophole in some wish-induced immortality, my character literally was one of the Big Bads. I went back in time, killed her before she became immortal, and impersonated her for 1500 years. To maintain the integrity of the timeline I had to do all the stuff the Big Bad had already done up to the present day, but as soon as the clock struck "Now" I (as that Big Bad) flipped sides to join the party.
I brought an army of minions and plenty of intel about the other Big Bads along, too. Quite a fun bit of plot judo.