The Bloodmoon Prophecy is a legend of the Skaal, a native group of Nords from Solstheim. It foretells an event which happens every era, when Hircine walks Nirn with his Hounds to collect mortals that shall become quarry in the Great Hunt. The selected are either taken to the Hunting Grounds, or preyed upon on Nirn.
When the Great Hunt is to occur on Solstheim, 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 that 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 is the Skaal's name for the Great Hunt, 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 lead to the Great Hunt, which 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 Great Hunts nor the Bloodmoon are limited to the island of Solstheim. A Great Hunt is also staged in the Chimera of Desolation, which is usually an entirely daedric affair. The Bloodmoon is known to appear on Tamriel wherever Hircine has designated a hunt.
By 4E 201, the Skaal's Bloodmoon Prophecy was seemingly a thing of the past. The werewolves of Solstheim had been reduced to only a 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.