Blood of most vertebrates contains hemoglobin, which contains iron. I've imagined a world where someone once had a seemingly stupid idea of using blood as a source of iron.
Now, the world is analogous to Earth pre-firearms Middle Ages (but with magic and dragons) and iron ore availability is the same too, i.e. quite enough. However, it turned out the blood iron has magic properties, which the usual metals most unfortunately lack. Actually, before this invention no one was able to enchant any mundane weapon or piece of armor, and battle magic as a whole is nonexistent. The mage-smiths who create this weapons are called "bloodforgers".
For now, the blood iron is mostly used to create swords and daggers. Its properties depend on source creature; each case needs unique spells to unlock the hidden properties. I can imagine three most important cases:
- Dragon blood. The dragon must be slain in honest heroic battle. The sword will be fiery and will easily hack through thickest armor. Even a frail teenager can easily split a full-plated knight with its horse in two with it, and they will be instantly roasted to "well done".
- Blood of innocents. Innocents should better be humans or unicorns, but puppies will suffice for weaker effect. They must be brutally killed, no mere bloodletting will do. The blade is no better than usual to cut through armor and bones. But when it touches flesh, it bites into it on itself, leaving deep wounds, and the bleeding is very hard or (if the victims were innocent enough) to stop. Favored by assassins.
- Wielder's blood. A dedicated warrior "donates" their blood repeatedly to create a weapon which is literally and extension of the arm. It feels the owner's intention and strikes with supernatural precision, it can even bend and become thinner to pass through the smallest holes in armor, and then it aims to the most critical organ within its reach, so it would never miss the heart or major artery by couple of inches.
The bloodforgers are trying to unlock any special properties in blood from such abundant sources as fallen warriors left on battlefields and slaughtered farm animals, but without success for now.
(Skip until here if you're not into fluff)
And now the hard part. I need to calculate how much blood is needed for a sword. No magic here, iron atoms won't pop out from nowhere.
This needs the following values:
- Mass fraction of iron in hemoglobin
- Hemoglobin content in blood (mass of hemoglobin per volume of blood)
- Total volume of blood in the body, or, for case 3, the amount of blood which can be donated safely per year.
Humans have the same physiology as in real world. Smallest dragons are of full-grown T. rex's size, largest - of Argentinosaurus's. Hemoglobin count in dragons is similar to large land mammals, or, more precisely, giraffes. Unicorns are just horses with a horn.
So, how many dragons or innocents do I need to slay, or can I donate enough blood in my lifetime?