This horn appears to be a normal trumpet.

It can be sounded as a normal horn, but if the command word is spoken and the instrument is then played, it deals 10d6 points of sonic damage to creatures within a 40-foot cone and causes them to be stunned for 1 round, and deafened for 4d6 rounds (a DC 19 Fortitude save reduces the damage by half and negates the stunning and deafening). Crystalline objects and creatures take 16d6 points of sonic damage, with no save unless they’re held, worn, or carried by creatures (Will DC 19 negates). If a greater horn of blasting is used magically more than once in a given day, there is a 20% cumulative chance with each extra use that it explodes and deals 10d6 points of damage to the person sounding it.

Prerequisites: Craft Wondrous Item, greater shout.
Cost to Create: 35,000 gp, 2800 XP, 70 days.