The Horn, also known as the Horn of Quin, is an Artefact of the Elder gods. The Horn was an early discarded mouthpiece used by the Elder gods to communicate and control the TokHaar. It was the Elder gods' most premature form of communication to the TokHaar, since it was not possible for them to use their own voices without eviscerating any and all things in their vocal range. As time went on, the Elder Gods began to refine and evolve their communication methods, and thus, the Horn's purpose became obsolete. It was eventually discarded just like many of the other Elder Artefacts, replaced by newer forms of communication, such as the "Kra's" in Heart of Stone and the spawned Voice of Jas' seen in Sliske's Endgame.
The Horn was eventually found by Guthix on Gielinor in the First Age. It was likely one of the many artefacts that granted him the power he was known to have. Along with the other artefacts in Guthix's possession, it was hidden by him before he went to sleep at the beginning of the Second Age. Around year 149 of the Fifth Age, it came into the possession of Quin, a powerful Sea singer and self proclaimed goddess of the Wushanko Isles. She used it to boost her already potent sea singer abilities, and used it to control all the Sea monsters of Wushanko from afar, and demanded that the citizens of Wushanko worship her like a goddess.
The basis of the Horn's ability is to communicate and impose one's will on another, which is why Quin was capable of controlling multiple Sea monsters.
Prior to the reveal that it was in Quin's possession there was was a lot of speculation that it could be the Frostenhorn or even the Wand of Resurrection. These were however quickly confirmed to be false, while there was often a "no comment" on Quin's horn.
Marimbo ascended due to prolonged exposure of the horn[source needed].