Horns of Orcus

The Horns of Orcus are the actual horns from the undead god Orcus.

Background
The Horns of Orcus were originally a part of the god of undead, Orcus. In the history of Critical Role, 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 one of the horns sealed within Vasselheim. Over time, the Beholder 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, and used its power to begin forming an army deep within the Underdark to wage war on the surface world.

Additional Information
Much of the Season One storyline revolves around this horn as K'Varn used its power to force Illithid and Duergar to work together in preparation for an attack on the surface world.

The Horn of Orcus K'Varn possessed has been sealed away by Vox Machina once again in Vasselheim.

Powers and Abilities
The horns for a large part are still a mystery as to what they are capable of. 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 whoever the horn is attached to.
 * Attaches to any creature that physically touches it.
 * Cannot be destroyed by known means
 * Capable of reanimating the body it is attached to should the creature die.
 * Frost Damage
 * Naturally corrupting in nature.