The Glade of Dreams is once again under threat from the nefarious Mr. Dark, and once again, Polokus, the Bubble Dreamer, has called upon Rayman to save the day. From Polokus, Rayman learned that the only way to defeat Mr. Dark was to find the lost Horn of Lums.
—"Lore" section of Rayman's in-game description, Brawlhalla

The Horn of Lums is an object that is only mentioned in the fighting game Brawlhalla, and never appears in any Rayman game.

While the Horn of Lums is never shown visually, Rayman's description in Brawlhalla sheds some light on it. Its name suggests that it is connected to the Lums somehow. It also seems to hold extraordinary power, as it is one of Mr Dark's only weaknesses. Rayman's description in the game claims that Polokus sent him out to find the Horn when Mr Dark attacked the Glade of Dreams once again. He ended up finding the Horn as one of the prizes in the Grand Tournament of Valhalla, which caused him to join the tournament without a second thought. It is unknown if Rayman ever retrieved the Horn of Lums.

Trivia

  • In Rayman Legends, there are two gray, winged horns that have Lums pouring out of them at the end of A Cloud of Darkness!, the game's final boss level. It is unknown if these have any connection with the Horn of Lums.