Silver Horn of Valhalla is a rare wondrous items in D&D 5e. It does not require attunement.
Wondrous item, rare
You can use an action to blow this horn. In response, 2d4 + 2 warrior spirits from the Valhalla appear within 60 feet of you. They use the statistics of a berserker. They return to Valhalla 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.
The summoned berserkers are friendly to you and your companions and follow your commands.