A Cascade Monster (Japanese: カスケードモンスター Kasukēdo Monsutā) is a type of Monster Card. The color of their card frame is cyan, with a pattern of faint concentric rings in the background with various orbs dotted around the different ring layers. These cards are placed in the Extra Deck.
Like other Extra Deck monsters, Cascade Monsters cannot exist in the hand or the Main Deck (even with card effects). If a Cascade Monster would be moved to the hand or Main Deck, it is returned to the Extra Deck instead, unless a card like "Transmigration Break" or "Degenerate Circuit" is active, in which case the Cascade Monster would still be banished. Also like other Extra Deck monsters, if a Cascade Monster is not properly Cascade Summoned before being sent to the Graveyard or banished, it cannot be Special Summoned from there (unless it specifies another way to properly Special Summon it).
Cascade Monsters do not have Levels, Ranks, or Link Ratings, and instead have a Class. A Cascade Monster's Class determines the range of consecutive Levels that can be used to Cascade Summon it. A Cascade Monster's Class is depicted by a row of yellow stars in white orbs (as opposed to the yellow stars in red orbs used to depict a monster's Level, or the yellow stars in black orbs used to depict an Xyz Monster's Rank) that extend outwards from the center of the card (as opposed to from the right-hand side of the card as with Levels, or from the left-hand side as with Ranks).
Cascade Monsters do not possess Levels, Ranks, or Link Ratings. Therefore, Cascade Monsters cannot be used as material for a Synchro, Xyz, or Cascade Summon, and are unaffected by cards that modify or check Levels/Ranks/Link Ratings, such as "Harmonic Waves", "Feedback Warrior", "Gravity Bind", or "Level Limit - Area B".
In particular, since most Ritual Spell Cards require Tributing monsters whose total Levels equal or exceed a specific value, Cascade Monsters normally cannot be used for Ritual Summons; however, Ritual Spells that depend on ATK (such as "Meteonis Drytron") can use Cascade Monsters.
A Cascade Summon is the act of Summoning a Cascade Monster from the Extra Deck using 2 or more monsters with consecutive Levels as Cascade Materials after activating a cached Cascade Spell Card. The consecutive Levels of the Cascade Materials must approach (from above or below) or include the Cascade Monster's Class.
During the Main Phase, the turn player can Cascade Summon a Cascade Monster from the Extra Deck with a Class that is either approached by (from above or below) or equal to one of the Levels of the Cascade Materials. When a Cascade Monster is Cascade Summoned, any Cascade Materials with a Level less than or equal to the Cascade Monster's Class are sent to the Graveyard, while Cascade Materials with a Level higher than the Class are attached to the Cascade Spell Card used in the Cascade Summon.
Cascade Monsters also have a property called a Limit. The Limit of a Cascade Summoned Cascade Monster is equal to the Level of the highest-Leveled Cascade Material used in the Cascade Summon, minus the Class of the Cascade Monster, plus 1. It can also be interpreted as the number of cards that are not sent to the Graveyard following a Cascade Summon, among both the Cascade Spell Card and the Cascade Materials. Cascade Monsters that are Special Summoned by means other than Cascade Summon, or are flipped face-up after being face-down, are Limit 0, unless a card effect specifies otherwise.
Cascade Monsters have a high degree of flexibility in how they can be Cascade Summoned, as multiple different combinations of consecutive Levels can be used to Summon them, and likewise, given any two or more monsters with consecutive Levels, Cascade Monsters of multiple different Classes may be Summoned.
Although they also require a Cascade Spell Card in addition to monsters with consecutive Levels, the fact that Cascade Spell Cards are cached from the Extra Deck, rather than being in the Main Deck, makes them less taxing on card economy compared to the similar Ritual Spell Cards, and they do not have to be searched out beforehand. Most Cascade Spell Cards also have fairly easy reservation requirements.
Furthermore, if a player Summons a Cascade Monster using monsters with Levels higher than the Cascade Monster's Class, those monsters will be attached to the Cascade Spell Card used in the Summon as materials. These materials can later be Special Summoned from the Cascade Spell Card by following the same requirements used to cache that card from the Extra Deck.
In addition, Cascade Monsters often have effects relating to their Limits, with their effects becoming more powerful or flexible when their Limit is higher. Various support cards for Cascade Monsters also grant larger benefits for higher Limits. Combined with the aforementioned ability to Special Summon higher-Leveled materials back to the field, there is an equal incentive for players to Cascade Summon lower-Class monsters as well as higher-Class ones.
- Cascade Monsters, as well as Cascade Summoning, are heavily based on various concepts found in quantum mechanics.
- The concentric rings in the background of a Cascade Monster's card frame and the orbs in them resemble the Bohr model of the atom. The icon used for Cascade Spell Cards also resembles this.
- When a Cascade Monster is being Cascade Summoned, the duelist is said to "entangle" the materials and the Cascade Spell Card being used. This references the phenomenon of quantum entanglement, where two particles become highly correlated with one another, such that determining the property of one particle instantaneously determines the corresponding property in the other.
- The different Limits that a Cascade Monster can take depending on its Cascade Materials could be interpreted as being similar to the energy levels of a particle, such as an electron in an atom.
- Likewise, the fact that a Cascade Monster only has a Limit when Cascade Summoned, but not before, could be interpreted as that Cascade Monster being in a state of superposition between all of its possible Limits. Once a Cascade Summon is performed and the Cascade Monster is brought out, the Limit is "collapsed" into a single, defined Limit.
- Even Heisenberg's Uncertainty Principle can be seen in the relationship between Cascade Monsters and their possible Cascade Materials. Given any two or more monsters with consecutive Levels, there is no way to know for sure which Class of Cascade Monster the player will Summon, but what is known for sure are the Levels of the materials. On the other hand, if someone observes a Cascade Monster on the field, but did not know how it was Summoned or what its Limit is (perhaps by spectating the middle of a duel), then while the Class of the Cascade Monster is known exactly, it is not known what Levels the materials used would have been. A tradeoff between the certainty of the Levels of the Cascade Materials and the certainty of the Class can be seen in this context.