Many Planeswalkers took oaths to defend and protect the inhabitants of the multiverse. To fight each other, these oaths will be broken.
In Oathbreaker, each 60 card singleton deck is led by a Planeswalker designated as that deck’s Oathbreaker. Each deck has a Signature Spell that falls within the Oathbreaker’s color identity. Both of these cards reside in the command zone. The Oathbreaker variant uses all the normal rules for a Magic game, with a few additions. You can also begin with the quick rules.
The Oathbreaker format was created by the fine folks over at Weirdcards.org. It was “fully” developed in 2017, and has an active subreddit and growing fan base. Weirdcards is a charitable social club founded to bring friends and families together through the love of playing Magic. Besides creating Oathbreaker, Weirdcards regularly runs Magic events that raise money for a variety of charities, and helped to create the Magic-based charity, MagiKids.