The Bloodmoon Prophecy is a legend of the Skaal, a native group of Nords from Solstheim. It foretells the Hunt of Hircine, an event which happens every era when the Prince enters Mundus and releases his Hounds upon the land. The Hunt is proceeded by four signs. The first is the coming of the "Hounds", when werewolves appear on the island. The second is the "Fire from the Eye of Glass", a pillar of fire which appears on the surface of the frozen Lake Fjalding. The third is the "Tide of Woe", a horker massacre, the bodies of which then wash up all along the island's northern coast. The final sign is the namesake of the prophecy: the "Bloodmoon", when Secunda turns crimson from the blood of the Hunter's Prey. After the four signs is something called the "Hunter's Game", which varies from era to era. The game might involve the hunting of an entire tribe, or just one man. After the hunt, Hircine returns to his realm for another era.
The most recent Bloodmoon Prophecy was in 3E 427. It took place within the Mortrag Glacier. Hircine summoned the island's three champions to take part in the hunt: Karstaag the Frost Giant; Captain Falx Carius of Fort Frostmoth; and Tharsten Heart-Fang, leader of the Skaal and wielder of Hircine's Ring. It is unknown if the Nerevarine took part in the Hunt as a Hound or as Prey. The Prey had to navigate a maze filled with werewolves to reach the Hunter. The Nerevarine made it through the maze, but in order to escape the Hunt it was required to fight an aspect of Hircine (Strength, Speed, or Guile). The Nerevarine succeeded in defeating the Hunter, causing the glacier to collapse and ending the Hunt of Hircine.
Neither Hircine's Hunts nor the Bloodmoon are limited to the island of Solstheim. Hircine stages the Wild Hunt in the Chimera of Desolation, which is usually an entirely daedric affair. The Bloodmoon is known to appear on Tamriel where ever Hircine has designated a hunt.
By 4E 201, the Bloodmoon Prophecy was seemingly a thing of the past. The werewolves of Solstheim had been reduced to only the few members of the Frostmoon Pack. Their alpha, Majni, still held the remaining blessed rings from their past hunts. The island also saw a rise in the number of werebears, who were laying claim to werewolf territories.