Horns of Orcus

The Horns of Orcus are the two actual horns from the demon lord, Orcus.

Background
The Horns of Orcus were originally a part of the demon lord of undeath, Orcus. Thousand years ago, two of the horns were cut off by Orcus himself and given to an avatar to be brought into the Material Plane as a way to open a doorway between the two dimensions. This avatar fought hundreds of unnamed heroes, and many fell to the creature's wrath. Eventually, it was destroyed, and the horns were sealed away.

At some point, K'Varn the beholder became aware of the location of one of the horns. Over time, K'Varn was able to pull the horn from the Material Plane into the Far Realm, where he merged with the evil artifact. He then made his way to the Material Plane, whether by his own will or Orcus's is unknown, and used its power to begin forming an army deep within the Underdark to wage war on the surface world.

Chapter 1
K'Varn possessed the Horn of Orcus, he used its power to force illithid and duergar to work together in preparation for an attack on the surface world. When Lady Kima's vision quest led her to the Underdark to investigate a great evil, she was captured. Lady Allura began to worry and asked Vox Machina to find and rescue Kima. Eventually, Vox Machina encountered and defeated K'Varn, recovering both Lady Kima and the Horn of Orcus, sealing the horn in Vasselheim beneath the temple of Bahamut, the platinum dragon.

Powers and Abilities
The horns are, by and large, still a mystery, even as to their capabilities. Kima stated that, had K'Varn managed to unlock the full power of the horn, Kraghammer would have been a ruin before Vox Machina had arrived.
 * Able to enhance the powers of whomever the horn is attached to.
 * Attaches to any creature that physically touches it.
 * Cannot be destroyed by known means.
 * Capable of reanimating the body to which it is attached should the creature die.
 * Cold damage on contact.
 * Naturally corrupting in nature.

Trivia

 * During the Campaign Wrap-Up, Matthew Mercer was asked what was the final fate of the second Horn of Orcus. However, Matt refused to answer, implying that the Horn might make an appearance in the second campaign.