The Horn of Valhalla is a mythical magic item that summons berserk viking warriors from the Halls of Valhalla to fight for the person who blew the horn (this is a common role-playing game magic item).
These horns usually appear as a normal instrument, and will usually have a command word inscribed upon them. The horn will only work if the command word is spoken before the horn is blown (some individual horns will also simply not work for certain kinds of people).
All of these horns can only be used once per week, and some of them gravitate towards good, evil or some other philosophy, and will only summon warriors with the same beliefs (these warriors will attack the summoner if they have a radically different philosophy). The summoned warriors will remain for one hour, and will fight until dead.
All summoned warriors will wear chain mail or banded mail armor, and will carry either sword, battle axe or spear (with most warriors having a weapon in each hand).
There are four known varieties of this horn.
The Horn of Silver
The Horn of Silver can be used by anyone. It will summon 4-10 berserk warriors of average ability. This is the most common variety of the horn.
The Brass Trumpet is a little more uncommon. This one will summon 3-9 experienced warriors. This horn is notable in the fact that it will not function if the wielder is a warrior himself (spellcasters, normal men, and rogue types can use this horn without problem).
The Bronze Summoning Horn
The Bronze Summoning Horn is fairly rare. This particular instrument can only be played by warriors and priests (of any devotion). It summons 2-8 powerful, battle hardened warriors (who are each powerful enough to take on a whole squad of normal men).
The Ironhorn is the rarest of all the Horns of Valhalla. This one can only be played by a mighty warrior. It will summon a small band of 2-5 warriors, who will be truly mighty heroes in every sense of the word.
This is only my interpretation of this item, which is based largely upon its use in the older versions of the Dungeons & Dragons series of games. Your Horn of Valhalla may vary, so always read the instructions.