Demon ichor

Demon ichor was a combination of blood, fluids, organs, and other internal remains of a deceased demon.

It was found in large pools on the first layer of the Nine Hells, Avernus, as that served as the main battleground of the Blood War, where many demons were regularly slain.

Properties
The ichor of demons was a sticky, adherent substance that seemingly resembled bile or thick tar. It was typically black in color.

While the substance was largely harmless on many planes of existence, on the lower planes it had several transformative properties. If it came into contact with certain beings, aside from fiends, plants, and undead, demonic ichor could change their physical properties. Changes included increasing or decreasing in size, either wholly or a single appendage; the growth of eyes, horns, or a tail; or something as simple as a new hair color.

Applications
Many devils and other fiendish creatures used demon ichor for certain rituals or as a means to improve their infernal war machines.

Still others used it as a coating for their weapons. When a creature was struck by a weapon treated with demon ichor in the lower planes, they suffered the same transformative changes as if they were exposed to the substance in any other manner. It also acted as a poison that caused weakness.