|Appears in (anime)||
Union monsters (ユニオンモンスター Yunionmonsutā) are Effect Monsters with the ability "Union". They each have an effect that allows them to equip themselves to a monster (as an Equip Spell Card) or unequip themselves to return to the field.
Cards such as "Worm Millidith", "Dark Necrofear", "Kiseitai", "Armory Arm", and "Number 58: Burner Visor" act as Equip Spell Cards but do not have the text designating them Union monsters; the latter two of the aforementioned five can even become monsters again.
Older Union monsters destroy themselves if the monster they are equipped to would be destroyed by battle while equipped. More recent Union monsters destroy themselves if the monster they are equipped to would be destroyed by any means.
Union monsters can either equip or unequip to the host once per turn with their own effect, given they are on the field. Additional cards, such as "Formation Union" and "Combination Attack", also allow them to equip and unequip. Some Union monsters fuse with their hosts (e.g. "V-Tiger Jet" and "W-Wing Catapult" to form "VW-Tiger Catapult", or "Dark Blade" and "Pitch-Dark Dragon" to form "Dark Blade the Dragon Knight").
The most recent Union card text reads as follows:
Once per turn, you can either: Target 1 [X] you control; equip this card to that target, OR: Unequip this card and Special Summon it [in Attack Position]. While equipped by this effect, [...] (A monster can only be equipped with 1 Union monster at a time. If the equipped monster would be destroyed [by battle or card effect], destroy this card instead.)
However, the Union monsters introduced in Structure Deck: Seto Kaiba do not have the restriction of only one Union monster being equipped at a time. They also do not require you to Special Summon them in Attack Position after being unequipped from a monster.
- 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 ATK or DEF.