The weed-kraken of Nenuial was a nightmare,a slimy, many-armed monster, afraid of the sun, roused from the lake bottom only by fierce storms at low water, it stroke without warning and carried off innocent fisherman who ventured out in darkness. It could be hurt, and even killed, but ever it was born again. So said the legend.
The true key to this perennial problem was an artifact called the Urn of Anskemidese. Donated as a diplomatic gift to the Royal Library at Annuminas by a Gondorian Hando, it was dumped in Nenuial by King Arveleg's chief librarian in T.A. 1409, upon the approach of the Witchking's armies. Half-buried amidst debris on the lake bottom, the Urn had the bane of warping, in some fashion, whatever was placed within it. Periodically every 2-20 years turbulence trapped some small creatures inside the Urn long enough for its evil magic to take effect, and another monster was born.
The creature could be of almost any size or description: a motile lake weed with a score of bloodsucking fronds, a pike with a dozen hydra-like heeds, a multi-clawed crayfish the size of an oxcart; all have occurred and would occur again; the pattern alone was consistant. Since the Urn tapped the power of Morgoth, the Enemy of Life, the creatures created instinctively hated the natural life of Endor and were repelled by bright light. Those monsters that were too small to stalk human prey were eventually netted or fished out of Nenuial by Arthadan boatmen who spiked them to the sides of buildings as curiosities; the abominations that craved mannish flesh enough to brave the surface air at night or on gloomy days created mystery and horror until some brave or lucky soul put a spear through them. Ending the plague of weed-kraken was a matter of identifying the Urn as the source of the problem, locating it, and destroying it in an Elvish or Dwarvish iron smelter; fire alone couldn't do the job. The available clues lay in half destroyed records and half-forgotten memories scatterd across Arthedain; a shrewd researcher may have been able to differentiate between reports of the weed-kraken and those of the Twilight Dragon.