The Horn of Evil is a powerful and dangerous artifact. When blown, it emits an awful noise and a black cloud that fills the surrounding area, overpowering everything good it touches and turning everything beautiful and harmless it touches into something evil.
Clawful and Trap Jaw once stole the Horn of Evil from a vault on Trolla. They used it to incapacitate the Sorceress and gain entry to Castle Grayskull, but were thwarted by He-Man, Orko, and Dree Elle. Yukkers, however, accidentally blew the horn when he dropped his sneezing powder, causing it to activate. It summoned a cyclops and a titanic fist and turned the animals, birds, trees, and even rocks in the forest around Castle Grayskull into monsters, but the Sorceress told Orko and Dree Elle that the song on the horn could help. They found some sheet music and sang it together while He-Man blew into the other end of the horn, causing the black cloud to dissipate and negating the effects of its evil magic.