Source - Waterdeep: Dungeon of the Mad Mage

Wondrous Item, Rare

You can use an action to blow this horn. In response, 3d4+3 warrior spirits from the Abyss appear within 60 feet of you. They look like Minotaurs and use the statistics of a Berserker. They return to the Abyss after 1 hour or when they drop to 0 hit points. Once you use the horn, it can't be used again until 7 days have passed.

If you blow the horn without having proficiency with all simple weapons, the summoned berserkers attack you. If you meet the requirement, they are friendly to you and your companions and follow your commands.