Can I wield The Spiked Fist of Destiny...?

[Rynjin]

It's all part of the same rule is why. The explanation was that each character only has two "hands of effort"; main hand and offhand. A 2H weapon takes both and can't be combined with other stuff because reasons I guess. You can't Unarmed Strike either...but you CAN us all your Natural Attacks at a -5 because...also reasons.