Luna’s Horn was a weapon wielded by Pelias, the first Starborn Prince, during the First Wars. Pelias powers were woven into the Horn itself. The Fae forged it in their homeworld, named it for the goddess in their new one, and used it to battle the demon hordes once they made the Crossing. During the final battle of the First Wars, Prince Pelias and the Prince of the Pit faced each other. The two of them fought for like three fucking days until the Star-Eater struck the fatal blow. But not before Pelias was able to summon all the Horn’s strength, and banished the Prince of the Pit, his brethren, and their armies back to Hel. He sealed the Northern Rift forever—so only small cracks in it or summonings with salt can bring them over now. Pelias wielded the Horn until he died. The Horn can only be used by a Starborn fae.
The Horn was broken so that it’s power could no longer be used. Micah wanted the Horn so that he could summon an army to decimate the human rebels. Micah blackmailed Danika into stealing the Horn. However, Danika did not trust Micah so did not give him the Horn after stealing it. Instead, she crashed the Horn down and mixed it with ink. She then got Bryce really drunk and took her to the tattoo shop so the Horn could be tattooed on her back. Meaning no one could find it and only Bryce could use it, due to Bryce being a Starborn. However, Bryce was unaware that the horn was in the tattoo.
Micah injected Bryce synth so that it would heal and fix whatever was broken with the Horn inside her. This would mean that the horn would then be whole once again and could be used through Bryce. He did not intend for Bryce to survive the synth. He just wanted her to open portals to another world, so he could have an army to destroy the rebels. Micah was successful in his plan and the seven gates in Lunathion opened portals to Hel. Bryce managed to use her Starborn power and the Horn to close the portals and restore the city.