Crimson Snow is a Master Blade, and was the birth blade of Aki Ishikawa.
It is the sharpest blade in the world, capable of cutting virtually anything. However, the blade comes with a blood price. On the first day of every winter, the wielder is dealt a grievous slash.
Tai's mother carried out this "ritual" and survived it for over a hundred years.
As previously stated, Crimson Snow is the sharpest blade in the world. It has been seen cutting cleanly through boulders, through other blades, and even the air itself.
Crimson Snow is one of the few weapons capable of easily dispatching an Elemental, the extreme of this being killing a Firstborn with relative ease.