A weapon of multiplicity is a weapon that bears the runic of multiplicity. Such a runic can be found on any melee weapon that a runic can spawn on (which excludes broadswords, war axes, war hammers, flails, darts and javelins) and will only be found on a benevolently enchanted (blessed) weapon.
When a weapon of multiplicity strikes an enemy, there is a chance that one or more spectral weapons will be conjured to fight on your behalf. The weapon(s) conjured will have the same type and damage output as the weapon of multiplicity that created them but will be somewhat less accurate. Like all runics, the chance of the runic triggering is based upon the level of enchantment and type of the weapon. An axe or spear type weapon may trigger on each enemy struck, the chance is evaluated independently for each one. Whips can stall far away melee enemies as they will focus dealing with the spectrals first. Against immobile or unliving enemies like goblin/ogre totems, turrets or sentinels the runic effect will not trigger.
The number of spectral weapons summoned per hit depends on the weapon's effective enchantment level (the weapon's natural enchantment level plus the bonus/penalty from strength). Below +6, only one spectral weapon is summoned. At +6, 2 spectral weapons are summoned. And additional spectral weapon is summoned for every 3 enchantment levels above +6 to a maximum of 7 spectral weapons at +21.