Blood Moss

Blood Moss (BM) is an red-hued fungus which is prized by Britannian and  Serpent Islander magicians as a magical reagent.

Britannia
Blood moss was first described by Philpop the Weary, a court magician in service to Lord British in the early Age of Enlightenment. In this text, the fungus is described as an agricultural pest, growing only in excessively dry climes in direct sunlight, where it had but a fleeting lifespan - dying off within the span of a day as nightfall overtook its light source. It was explained by Philpop that this fleeting existence imbued the mushroom with a mystical potency related to quickness and movement, and the reagent has long been used to instigate such effects in Britannian spellcastings.

Curiously, blood moss in later ages appeared to be adapted to a wider range of habitats, and could frequently be found in damp marshy areas such as the Bloody Plains as well as in the forests of Spiritwood - implying perhaps that Philpop's description ultimately amounted to a well-intentioned fable. Over time, another explanation of the red fungus' origins developed, purporting that it sprouted on sites where blood had fallen. While such a history would well explain the prevalence of the reagent on the Bloody Plains, it falls short of providing as neat a cosmological explanation of the mushroom's mystic properties as Philpop. By the late Age of Armageddon, however, the legend of the blood had become accepted as fact, with scholars even taking stock in a new and lurid detail that the moss preferred the blood of virgins - purportedly leading to perverse abuses by amoral wizards in search of the reagent.

Serpent Isle
On the Serpent Isle, blood moss is a comparatively rare commodity, with the only known cache of the fungus growing in the swamplands of the Isle of Illusion, south of Moonshade. 

Trivia

 * The use of blood meal as an agricultural fertilizer may explain the difference in accounts regarding blood moss's habitat, as it does not seem impossible that crops grown from blood-infected soil might beget short-lived waves of the fungus. Nevertheless, it seems unlikely that the real world writers responsible for the varied literature regarding blood moss had a conscious reasoning for the shift.
 * Unlike many of the other reagents featured in Ultima, blood moss doesn't appear to have an immediately identifiable analogue in reality.