Rasp's Elixir

Rasp was infuriated! His fledgling army of undead were wasted in mere minutes! Their brittle bones and fragile, rotting flesh no match for healthy, armored soldiers, hiding behind their plates of metal and flashing steel weapons. He had barely managed to get out of the skirmish with his life, and now he had to abandon his current base of operations - a luxurious mansion with all the amenities - and retreat to a dank, soggy and downright smelly cave!

Rasp sat himself, hunched over on a rickety chair, and poured some wine in an intricate steel goblet as he pondered his failures. All his plans... His desires! How could he achieve control over this region if every time his army swells, it is torn asunder so easily? Aah, but therein lies the flaw, he realized. The risen dead - that which he had control over - was a most vulnerable creature. Even the most pristine bodies suitable for re-animation were inferior to one still alive. His thoughts turned from frustration to fury and with a scream, Rasp rose and threw his heavy goblet at a skeleton suspended on chains against the cave wall. Upon contact with the goblet, the skeleton's bones shattered and exploded with the weight of the metal receptacle causing a shower of bone fragments. Rasp stared at the sight as the bone-dust settled, and his mind clicked into place. Yes, bone could be crushed. Therein lies the flaw. Therein lies the solution. Rasp turned to his Alchemist table and got to work - this would take much trial and error...

First we will need blood. A lot of blood. Gallons of the stuff.
Yes... Boil it down; dry it up; concentrate it into saturated flakes of red.
Add it now, to the vial. Good, now stir in four drops of the acid - careful now! - we'll let that sit for a few hours.
A splash of mercury, shavings of Tin and Iron, Zinc and Silver, Mithril and Gold. Fire it up now; it must get very hot!
We must add the ether now... Don't spill any, this strain cost me more gold than I have!
Leave me be, now - i must perform the enchantments while it is still hot - I mustn't be disturbed for two days!
When you return, the draught will be ready for experimentation.

Months of failure, cost and effort passed before Rasp finally was able to perform the first test of his new Draught. And just weeks after he poured the first vial down the gaping jaw of a ragged zombie, he saw the results; everything he had hoped for was fulfilled. The ingredients were expensive. he would have to devise something to fix that problem. But it was a start. Rasp nodded to himself as he peered into the exposed chest of the zombie, studying its gleaming, reflective rib-cage. He would like to see a warrior try to crush these bones!

Potion effects This potion does not just give undead metal bones - it is more complex than that. The potion is made of two main components; the ingredients, and the enchantments. the ingredients are simple; precious metals, expensive organic components and a lot of concentrated human blood - The purpose of these ingredients are to make any physical undead creature who consumes this (or has it poured over them) associate the smell and taste of metal with that of blood; it effectively gives them metal-lust to go alongside their bloodlust. This metal-lust is not specific; and the creature will have just as much a taste for a delicious roll of aluminium foil, as it will for hardened Mithril.

The enchantment component of this potion is two-fold. Firstly, it slightly increases jaw strength and tooth density in a way to allow the crushing of metals with a lesser chance of fractures. Secondly, once the metal is consumed, it is slowly worked into the bones of the beast, converting bone to whatever metal has been consumed. It will take many weeks or a massive binge to convert the skeleton to metal - considering a human skeleton is about 25 pounds and since metal is denser, it will take more than that.

once the bones are fully metal, any more consumed will replace that which is currently in the bones, and any excess is shed as fine metal filings, 'sweated' out through the skin, giving a fully transformed creature a sickly metallic sheen.

One side effect of this potions effect is that there is a small chance that an undead attacking a person will attempt to take a bite out of their sword or armor, as opposed to the person. This has a higher chance of occuring if the undead is not under thrall of a necromancer, and is just wandering mindlessly.

Side notes

A lich could use this potion. Any physical undead could. consumption is fatal to living organisms

Imagine your party cresting the top of a hill to see an entire valley full of dead soldiers, Zombies crawling around like insects, feeding on broken armor and swords, and the odd Corpsemaker devouring broken cadavers whole - armor and all.(Front is just a mouth full of huge, scarily human-like blunt teeth. Truly a terrifying visual.

Undead augmented by this draught are harder to kill, but more valuable with scrap metal - there may even be some gold or silver in them.