The most free format of them all, with a miniscule R-List and no X-list. Open has a stigma of allowing infinite loops and broken combos, with strategies able to win in 3 turns and others able to generate impossibly large swarms. It has existed since the beginning of the game, but its notoriety grew with several combos released in Mount Doom and Shadows.
X- and R-List
Open has no X-list, and has an R-List consisting of only 6 cards: Forces of Mordor (1C248), Steadfast Champion (7U49), Memories of Darkness (10U2), Mordor Fiend (10C91), Strange-looking Men (11R100), and Orkish Smith (11C132).