Let's do the math...

You can donate three times a year to the red cross (once every 112 days). According to some page, you donate around 10% of your blood volume each time.

That means, for a volume equivalent of 300 people, you'd need to donate 3000 times, which would take around 1000 years.

The vampire is going to need more people, or to keep a person alive 1000 years with vampire magic.

Slaanesh approves

Or just mix in a few enemies with some normal iron.

• https://en.wikipedia.org/wiki/Longsword
• https://en.wikipedia.org/wiki/Composition_of_the_human_body#Elemental_composition_list

The numbers roughly check out. Not sure how much one can extract by just draining the blood though.

Someone else did the math, accounting for waste made during forging. https://www.wearethemighty.com/mighty-gaming/blood-iron-sword-myth-explored/

Steel requires only iron and up to about 2% carbon

Rest are minor alloying elements used mainly in modern steel alloys to improve the steel beyond what just carbon steel could do like for example stainless steels

1. Your link says these are elements commonly found in steel, not that they are all required. In fact it says of phosphorus and sulphur that they are generally undesirable.
2. We don't need to make a steel sword, an iron sword could do.

Either way you would definitely need carbon, but as you say that's pretty easy. I don't think any of the other elements are absolutely required.

Those are all of them, but that's for a lot of different types of steels. You don't have to have all of those metals to make steel. You really just need iron and a tiny bit of carbon. A few of your ingredients help with purity, and the rest are additives for different steel properties you may want. Like a touch of nickel for stainless steel.

You only need iron and carbon the rest is already alloyed steel. You can definitely make a good blade out of only iron and carbon, it won't be stainless, it might be difficult to harden just right, but it will be flexible and hold a keen edge if forged right. The smiths of ole dealt with nastier steels containing all kinds of things making it worse, not better (such as excessive amounts of sulphur and phosphorus) so I'd say they'd manage.

Somebody call NileRed asap

I want The Red Cross to hire you for marketing asap so this can actually happen.

imagine, over the centuries of blood donation, the sword slowly grows from a knife, into an absolutely huge dragonslayer behemoth

I'd suggest a warhammer mold, just for stability.

Mythbusters did this on their first episode: ice bullet. I think a sword might also be too brittle unfortunately.

Hold up... Wouldn't a diamond sword be better than a steel one?

New mythology curse just dropped.

And then you can apparently donate like 470 millileters every 8 weeks.

Safely. You could probably speed it up a bit if you have a higher risk tolerance.

New ethical dilemma just dropped - kill 300 to forge the sword, or deny 3150 people blood in an emergency to forge the sword...

New ethical dilemma just dropped - kill 300 to forge the sword, or deny 3150 people blood in an emergency to forge the sword…