[Basic Set] Invisible Weapon

A bit of silliness occurred* and resulted in the quandary: if hitting an invisible opponent can be a mere -4, what's the mod for hitting or defending with or against or striking at, invisible melee weapons?

*an attempt to illustrate tactical melee using 4" figures on 2" hexes; fig#1 leather hooded cloak, chain hauberk & trews, slung kite and a zweihander; fig#2 partially armored Obi Wanish figure with empty hands... invisible staff!
Plus four varicolored spidermen. Needs must:/

[Basic Set] Invisible Weapon