The "Lost Deck" (ロストデッキ Rosuto Dekki) is a different deck from your main deck which replaces a player's main deck after the use of the "Belost" ability of the "Lost World" flag.
- Lost Cards can only be put into a Lost Deck, and cannot be put into the main deck due to their different backing.
- Lost Deck consists of exactly 30 Lost Cards (cards with the black backing).
- A Lost Deck may only contain a maximum of 4 cards with the same card name (excluding the buddy monster).
- Even if a card has a different ability or illustration, the maximum of 4 copies is kept.
- If a card effect states that "This card is also regarded as X", only the name printed in the name box will count towards the "max 4 copies" rule (See also: Super Lethal Formation/Rulings).
- Both cards will have the same name, and are viable targets for any card that may support them.
- You may only choose a Lost World monster as your buddy if you have the "Lost World" flag in your main deck.
- A Lost Card Buddy is not flipped over and revealed at the start of the fight.
- When a player's flag becomes the "Lost World" flag, flip the Lost Card Buddy face-up.
- Face-down card in the buddy zone will be regarded as a blank card with no information. Buddy Call cannot be performed.
The removed main deck
- The main deck is removed from the game via the "Belost" ability
- All its cards face down and becomes a hidden zone and neither fighter can check its contents.
- Normal cards and Lost Cards cannot mix in the deck zone.
- When changing from the main deck to the “Lost Deck”, the normal cards in areas other than the deck area (field, soul, hand, gauge and drop zone) remain as they are, and the fight continues with the Lost Deck placed in the deck zone as a new deck.
- Normal cards and Lost cards can be mixed in zones other than the deck zone.
- For example, a player's hand may consist of both Lost Cards and Normal Cards.
- Card effects cannot return Normal Cards to the Deck if the currently used deck is Lost Deck.
- Such effects can be used, but the player using it may not choose Normal cards to be returned to the Lost Deck.
- For effects such as “drop one of your opponent’s gauge” or “choose a card from your opponent’s hand at random”, when choosing from a hidden zone where normal cards and Lost Cards are mixed, the cards must be chosen in a situation where it is possible to tell normal cards and Lost Cards apart.
- If not otherwise specified, you cannot prepare multiple Lost Decks, and switch a different one for each match during the same tournament.
- When using card sleeves, the sleeve for main deck and Lost Deck must be different in order to tell them apart.
- When preparing for the game, the fighter using a "Lost Deck" must place it in such a way that it is visible to the opponent.
- It is highly recommended to place it on the right of the drop zone.
- Tournament officials and judges may decide on the placement area of the Lost Deck, if it is deemed that there is insufficient space for it on the fight table.
- With acknowledgment from both fighters, a judge may rule for them to place the Lost Deck into a container such as deck cases or deck boxes, but care must be taken not to mix them up with other cards unrelated to the fight at hand.
- A fighter who wishes to use a Lost Deck must shuffle both the main deck as well as the Lost Deck during the preparation part before a game.
For purposes of differentiating, Bushiroad's staff has emphasized in certain terms like "White Deck" (白きデッキ Shiroki Dekki) and "Black Deck" (黒きデッキ Kuroki Dekki).