Scarlet Night

Scarlet Night is an extremely poisonous red-coloured fungus found in Opal territory.

Contact with Scarlet Night on bare skin will result in a swift and painful death. Victims of Scarlet Night are characterised by foam around their lips, blue faces, and blisters forming wherever the fungus made contact with the skin. However, if the fungus latches onto clothing, it will die in a couple of hours and become harmless to interact with. The dead fungus can then be used as a deterrent against various creatures, such as rats.

Secrets of Deltora
Doran the Dragonlover wrote about Scarlet Night in his traveller's guide, Secrets of Deltora.

City of the Rats
In order to escape from the city of Noradz, Lief, Barda, and Jasmine had to pass through a tunnel covered in Scarlet Night. They stole the outfits of the city's leaders, the Ra-Kacharz, and were able to pass through the tunnel unharmed, while Reece—the leader of the Ra-Kacharz who they brought as a hostage—was killed just from coming into contact with the fungus.

Later, when the companions reached the City of the Rats, the dead Scarlet Night clinging to the Ra-Kacharz's outfits protected them from the hoards of rats.

Anime
In the anime, Scarlet Night is green coloured. When it comes into contact with bare skin, it seems to rapidly change the victim's body into a colony of fungus.