Source: Yugioh Wikia
Fusion Monsters, colored violet, are cards representing a combination of two or more monsters. Fusion Monsters may only be included in the, and cannot exist in the or . If a Fusion Monster is returned to its 's or , it is instead sent back to the . However, they may be sent to the when , as well as .
Originally, the only way to properly summon awas through a involving the required cards as well as . Monsters used with are called . If a Fusion Monster is ed to the field via a card such as , and then sent to the , it cannot be ed to the field from the with cards such as , , or . In order for a Fusion Monster to be returned from the to the field, it must be ed properly.
There are some Fusion Monsters that have special. For example, the Fusion Monsters have a clause stipulating that they may not be ed unless they are being ed. This also rules them out from being ed either from the or when .
1 Fusion Material Monster can be replaced byin s with cards that either do not specifically state the monster to be used as a , or do not have a summon condition that limits either what monsters may be used for the , or in what way it may be summoned. Most Fusion Monsters need two Fusion Material monsters, and then the card. However, certain monsters such as only need the monsters on the field, and can combine without Polymerization.