Monster weapons are used in all Disgaea versions by monsters only; Human characters are not allowed to use them. In the same way, monsters can only equip monster weapons.
Monster weapons do not affect a character's sprites or animations: A Nether Noble will always be shown wielding an axe regardless of the weapon equipped.
Monster weapons cannot gain proficiency and give skills, unlike the other weapons. In later installments, they increase in power according to the monster's level rather than their Mastery, since Monster Weapons cannot gain it.
Disgaea: Hour of DarknessEdit
Disgaea: Hour of Darkness had two unique monster weapons that were listed as ranks 41 and 42, going beyond the usual 40 rank barrier. The two items could only be stolen from Baal and Prinny Baal, respectively. In Disgaea DS, the rank 41 weapon Nemesis can actually be acquired by beating certain bonus bosses, since they come equipped with it once they join the player's party... even if theirs was stolen during the battle.
To see the charts of weapon skills and weapon stats for Monster weapons, please see List of Disgaea 1 Weapons#Monster.
Disgaea 2: Cursed MemoriesEdit
Disgaea 2: Cursed Memories introduced a change from the original Disgaea, in that there were two kinds of Monster weapons: one kind aimed at pure attack, the other aimed more at intelligence.