I've been thinking about this. Here are a few rules suggestions for variant trumpets, organized by what their trumpets do. All these can be used once per day.


Breaker of Seals
When blown, the Breaker's Trumpet projects a sixty (or so) foot cone that has the effects of a Freedom spell and a targeted dispel magic that, however, only affects certain spells, namely those that seal or lock a portal or doorway (e.g. Hold Portal, Arcane Lock) or bar creatures (or certain kinds of creatures) from entering an area (e.g. Antipathy, Antilife Shell, Magic Circle against Alignment). The effect's caster level is (appropriate number).

Actually, thinking about it, that does sound almost a bit more like a chaotic than a lawful thing. Oh, well. Can't have everything.

Harbinger
When blown, the Harbinger's trumpet summons a unit of celestial warriors. 2d6 (appropriate intermediate level archons, probably elite warrior hounds) are summoned.

Thunderer
The Thunderer's trumpet, when blown, creates a (long range) cone that shatters all brittle objects as if they were affected by the area effect of a shatter spell. Additionally, a single solid object or building of up to (immense size) in the cone is destroyed as if by shatter's targeted effect. Finally, the cone deals (amount) of sonic damage to all creatures in the area. A fortitude save (DC appropriate) halves the damage.

Cataclysm
The cataclysm's trumpet, when blown, brings down the wrath of the heavens upon an area, creating a supernatural Storm of Vengeance effect centered on the Archon that lasts for 24 hours, with no concentration required from the Archon.