Is it true that blood strengthens swords? Like in Naruto, when Zabuza said that the iron in the blood of his enemies keep his cleaver even more powerful? Any truth to that?
This is an old myth and I've never been able to find the origin of it. The earliest primary source I've seen is from 1894 in a Berlin newspaper, long removed from any of the periods during which it supposedly happened. Classic versions include wootz steel blades from Damascus, viking swords, and katanas; often quenched in the body of a slave and sometimes in the blood of a dragon. Incidentally, while researching this I found a myth that blades warp if not quenched facing true north so... a lot of forging myths out there on the internet.
To go a bit more in-depth, quenching a blade in a human body would be a terrible idea for a number of reasons, notably the risk of hitting bone and the inconsistency of the quenching medium. A living body would be even worse because they would, understandably, thrash around. Quenching in blood could work and it does have a decently consistent salt content (which is the only thing I've seen mentioned in its favour) but it congeals and would go rancid. Urine, apparently, works better for less hassle. Urine also shows up in more historical sources, often with advice about how to get the right sort of urine. Regardless, oil or water are much more common, almost certainly better if you know what you're doing, and couldn't possibly smell as bad.
Also blood may contain iron but it also contains salt and oxygen, two things that aid in rusting. This shouldn't be a problem in a quenching medium but does make Zabuza's logic somewhat suspect. Of course in your world its your rules so the blood of certain creature could be the quenching medium of choice, especially if its magical. Alternate use for unicorns, maybe?
Another thing, I want your take on this. I have a country of knightmages in my story. They use broadswords with enchantments that make their weapons lighter and tougher. I also have a country of samurais. Their katanas are bespelled to not be as fragile as they are in real life. Have any of you ever used magic to bypass the fragility of swords?
I have one world where they put different sections of the enchantment on different pieces of iron then pattern-weld them together to form the final enchantment. This is done to sidestep complexity issues that arise when trying to enchant homogenous steel. It's rare, however, as I generally try to keep magic inconvenient for both plot and preference reasons.