|Appears in (anime)||
Union monsters (ユニオンモンスター Yunion Monsutā) are Effect Monsters with the "Union" ability. Each Union monster has an effect that allows it to equip itself to a monster as an Equip Spell Card or unequip itself to Special Summon itself to a Monster Zone.
Union monsters were introduced in Advent of Union.
Not all monsters that can treat themselves as Equip Cards are Union monsters; for example, "Worm Millidith", "Dark Necrofear", "Kiseitai", "Armory Arm", and "Number 58: Burner Visor" all have effects that turn these cards into Equip Cards, but none of them are Union monsters.
Every Union monster has an Ignition Effect that allows it to either equip to or unequip from the host once per turn while it is on the field. If a Union monster unequips itself with its own effect, it is Special Summoned; Union monsters last printed prior to Structure Deck: Seto Kaiba can only be Special Summoned in Attack Position if Summoned this way. Additional cards, such as "Formation Union" and "Combination Attack", can equip and unequip Union monsters, with "Formation Union" even treating the Union monster as having been equipped by its own effect. They also do not require you to Special Summon them in Attack Position after being unequipped from a monster.
Older Union monsters have an effect to destroy themselves if the monster they are equipped to would be destroyed by battle. More recent Union monsters destroy themselves if the monster they are equipped to would be destroyed by any means. All Union monsters printed since Structure Deck: Seto Kaiba (including reprints) destroy themselves if the monster they are equipped to would be destroyed by battle or card effect.
Prior to Structure Deck: Seto Kaiba, all Union monsters had a condition that meant that only one Union monster could be equipped to a monster at a time. In the OCG, from Structure Deck: Seto Kaiba onward, Union monsters no longer have this restriction (including reprints).
The most recent Union card text in the TCG reads as follows:
Once per turn, you can either: Target 1 [X] monster you control; equip this card to that target, OR: Unequip this card and Special Summon it. A monster equipped with this card [...], also if the equipped monster would be destroyed by battle or card effect, destroy this card instead.
- Newer Union monsters tend to combine with Machine-Type monsters and are Machine-Type themselves.
- Currently, there are no Union Monsters with more than 2000 DEF.