So, I have a silly idea for a story, and when this happens, it usually bleeds over into game material. I won't bore people with all the particulars of this story, but one of the components of it is giving me a bit of trouble; on this war-torn planet, where the Imperium has been fighting against the Orks for decades, there is a high-ranking IG officer (I've never been so great with ranks, so exactly where on the chain he falls is iffy, but he's whatever the likely head of most of a world's military would be, upon assuming command of said forces, after arriving from Cadia), and he carries a relic powerfist, referred to as the Fist of Rule. The problem is, this "general" is actually a Chaos Cultist; much of his standing command staff is, too, and the "Fist of Rule" is actually the Choking Fist, a relic tainted by Nurgle. Between growing up in constant war with the worst the Warp can spew at a planet, and then fighting Orks endlessly, for years, the stagnation of the situation, and the slow decay of the Guard, and the Orks, seems like a sign of Nurgle (humorously, I said i wouldn't belabor with story, and I'm still sort of not ). The Choking Fist is a powerfist either possessed of Nurgle's taint, or, hopefully possessed by one of Nurgle's servants.

What I am actually looking for is a way to reflect that the Choking Fist/Fist of Rule is a weapon that actually withers its targets. It is renowned as being especially potent against tanks, and fortifications, though, in truth, it is the power of Nurgle spreading into said targets, and decaying them, and most wouldn't realize this, on account of the ramshackle quality of Orky construction. Short of just having the Fist do more damage, is there a good way to reflect the entropic effect it has on what it hits? So far, I am not certain if it has a more deleterious effect on people/Orks, themselves; its power might not be awakened enough, yet, though Nurgle is looking forward to contaminating the Orks with his sickness, eventually.

Kind of a weird one, I know. More Armor Pen? Maybe decrease Armor value after each attack? I don't want it to seem pointless, after it overwhelms the Armor, which doesn't further add to the damage, but I'm actually not looking to make an entirely cheesy weapon. Thanks much for any ideas.

I would think the Corrosive quality is just what the plague doctor ordered. You could simply increase its value as the power grows/awakens.

And be sure to check out the Daemon Weapon rules in Enemies Beyond or even the Legacy Weapon rules in Tome of Bloo d! (Shameless plugs.)

The Corrosive quality is definitely the way to go for what you've described.

The Toxic quality would look like another trait you might use (not for tanks and fortresses, of course)

you could also have it act like a rad grenade if used as part of a charge

Yes as stated before: Corrosive is the keyword. Decay (BC Tome of decay) and Toxic are also good, but nothing beats corrosive for what it's suposed to do.

Oh and please have the cultist shrine contain a banner or a mural of the wielder of the Choking Fist strangeling a Lord of Change with it! (You know: Choking the chicken?! ;) )

To add to everyone else, Crippling may also be a useful quality if you want the powerfist to have a greater effect on living beings. Crippling is also handy if you want the weapon to scale up and become more powerful as Nurgle's influence continues to grow.