From Lost Fables Wiki
A Hydra’s blood is highly corrosive, and relatively colorless, though when exposed to oxygen during collection; it bellows a dangerous orange smoke while the liquid itself adopts a slight yellow tint. If it makes contact with the skin, the corrosive liquid would destroy the nerve cells; the victim would not feel the pain of even their bone being eaten away. As such, the blood is highly toxic in any state (gas, solid, liquid), and requires dilution from a trained alchemist for any practical use. When diluted, it is known to have many uses, from engraving to the creation of agricultural fertilizer.
Gathering the blood of a Hydra is no easy feat, as it would take a party of adventurers to take down even an adolescent creature of this nature. Furthermore, the collection of the blood is considered more dangerous than fighting the beast, as the blood fumes with a toxic smoke upon exposure to the oxygen in the air, requiring preparation in advance. When the blood is finally collected in a glass vessel, it must be air-tight and secured away from heat.
The standard use of Hydra blood is by craftsmen for using a diluted mixture to add intricate details to their works, such as artificially aging pine and maple wood, or engraving fine illustrations upon their designs. Furthermore, tailors have been known to use large amounts of diluted blood for the creation of sophisticated dyes suited towards the upper-class and nobility of their lands. There is also the rogue story of those using undiluted portions of the blood to dissolve bodies.