The Justarmament monsters are an archetype of Armor monsters used by Illdoon. They differ from the Armor monsters used by Valon in that, rather than being parts for a high suit; they are different forms of weapons, from swords and spears to pistols and rifles. Rather than being summoned to individual spaces on the field, the monsters appear on the person of the duelist (either on their back; attached to their hips, arms, or legs; or on/in their hands). Like Valon's monsters, only one Armor monster can attack per turn; but unlike them, if the monster the player attacked with is destroyed, any other attack becomes a direct attack regardless of if their are other Armor monsters on the field. The only times this is not the case is through the effects of certain Spells and Traps, or through the archetype's main aces Number 112: Full Armor Justarmament - Blade of Destiny and Number C112: Chaos Armor Justarmament - Dark Destiny Blade, which are themselves full suits of armor worn by the duelist.

Most of the Justarmament monsters, mainly those based on swords, spears, clubs, and the like; gain certain amounts of ATK based on the duelist's Life Points, reflecting the strength of the duelist's swing behind that weapon. Other Justarmaments, those based on firearms and lasers, has set amounts of ATK and some can attack multiple times. After the attack(s), though, their ATK drops to 0 until the next turn's End Phase.

A majority of the Justarmament monsters are inspired by the Weapons cards from Future Card Buddyfight.

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