This is a common magical item in several fantasy role-playing games.
The Horn of the Tritons is an enchanted conch shell that is used by an undersea race known as the Tritons (they have their own node, so go ahead and read that first, this node will still be here when you get back). This shell differs from the non-magical conch shells that many Tritons carry, only noble Tritons or those of great power will have one of these horns.
The horn is always a large conch shell, usually an absolutely perfect and beautiful specimen. It has been enchanted in a secret ceremony by a powerful Triton mage. The enchantment gives the horn several powers related to the sea.
Anyone can use this horn. Humans can use it once per day, while it can be blown three times per day in the hands of a Triton. It can have any one of three different effects (the effect will be random when blown by a user who is unaware of its powers).
The first power is to summon a squadron of undersea creatures to come to your aid. These creatures will be friendly, and will obey the horn wielder as if they were a trusted friend. Giant sea horses, Hippocampi, and sea lions are the creatures that will most commonly answer this call.
The second power is to create a complete calm over up to a mile of water. This has the side effect of slaying any creatures that are composed totally of water (water elementals mainly).
The final power is to cause a total panic to come over most normal marine animals. They will flee immediately, and will not return for several hours. This effect does not work on dolphins, who are too intelligent to fall for the trick.
Although anyone can use this horn, the Tritons consider it to be one of the treasures of their race. Any Triton within 3 miles can hear the sounding of this horn, and they will always respond. They will expect any non-Tritons to give up the horn to them (otherwise they will attack).
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 the Tritons may vary, so always read the instructions.