[…No tales describe what form the nuckelavee takes when in the sea, but its appearance on land has been recounted in graphic detail. An islander, Tammas, claimed to have survived a confrontation with the beast and, after much cajoling from Dennison, reluctantly gave his description of the monster, the only known first-hand account.
The monster described by Tammas has two heads; the equine head has an enormous gaping mouth that exudes a pungent, toxic vapour, and a single giant eye like a burning red flame. A particularly gruesome detail is that the nuckelavee has no skin; black blood courses through yellow veins, and the pale sinews and powerful muscles are visible as a pulsating mass.
Other reports state that the creature resembles a centaur; narratives are inconsistent in the finer details of the demon's description however.
Traill Dennison only describes a man's head with a "mouth projected like that of a pig". Marwick also only mentions one head with a single red eye, and he borrows some of Tammas's characterisation by recording the creature's mouth as "like a whale's".
The nuckelavee's breath was thought to wilt crops and sicken livestock, and it was considered responsible for epidemics and drought.
Seaweed burning to create what was known at the time as kelp began on Stronsay in 1722. The product – soda ash – was an alkali mainly used to treat acidic soil, although as time went on its commercial importance in soap and glass manufacture increased.
The pungent smoke emitted during the process was believed to enrage the nuckelavee, resulting in a wild rampage of plague, the deaths of cattle and the destruction of crops.
The nuckelavee was said to have infected horses on Stronsay with the deadly disease known as mortasheen, to demonstrate its fury and exact its revenge against the islanders for burning seaweed; the infection subsequently spread to all the other islands involved in the industry.
The creature was also blamed for prolonged periods of abnormally low rainfall…]