War Horn

War Horn is an item which have a chance to drop after successfully defending an Item Island.

The War Horn can be given to your commanders in order to give them the ability to immediately call soldiers from a house of your choosing on an island. Level 1 War Horn calls 3 soldiers and can only be used once per island. After being upgraded to level 2 (8 coins), 5 soldiers are called instead of 3. Units cannot be reinforced above their limit of 9 soldiers. At level 3 (14 Coins) the War Horn gains the ability to be used twice an island, after a few seconds cool down.

As with every item in the game, once it is assigned to a commander, it cannot be removed anymore. You can choose War Horn as a starting item once you have unlocked it.

Link
*Source: http://badnorth.wikidot.com/war-horn