The help horn is a possible reward from the Reward Box in the Dominion Tower. It can only be used within the Dominion Tower Arena. To obtain the help horn, you must first reach floor 15 in Climber mode. Using the horn creates food on the ground in the arena for your use. When fully charged it has 5 uses. Upon using it, a piece of scorpion meat will appear on the ground near you. The charge will be shown in brackets after the name.
As each scorpion meat heals 2125 life points, a full horn equates to 10625 life points in one inventory space. This is the highest healing per inventory excluding stackable healing items. This makes the help horn power-up very useful in scenarios in which you need large amounts of healing in the tower, such as in endurance mode and some special mode challenges, in case Saradomin brews fall short.
It is also worth noting that deploying the help horn requires much more time and clicks than normal food items, and should not be relied upon for emergency healing.
Note: Given that the help horn counts as a power-up, it cannot be used in special modes or climber mode if the no power-up handicap is active.