Source: Dungeon Master's Guide

Wondrous item, rare (silver or brass), very rare (bronze), or legendary (iron)

You can use an action to blow this horn. In response, warrior spirits from the plane of Ysgard appear within 60 feet of you. These spirits use the berserker statistics from the Monster Manual . They return to Ysgard 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.

Four types of horn of Valhalla are known to exist, each made of a different metal. The horn's type determines how many berserkers answer its summons, as well as the requirement for its use. The DM chooses the horn's type or determines it randomly.

d100	Horn Type	Berserkers Summoned	Requirement
01-40	Silver	2d4 + 2	None
41-75	Brass	3d4 + 3	Proficiency with all simple weapons
76-90	Bronze	4d4 + 4	Proficiency with all medium armor
91-00	Iron	5d4 + 5	Proficiency with all martial weapons

If you blow the horn without meeting its requirement, the summoned berserkers attack you. If you meet the requirement, they are friendly to you and your companions and follow your commands.