Sounding a battle horn is a traditional way to signal the start of a massed attack. It's good because the signal carries a long way. This also means that the defenders will probably hear it too, which can go either way - perhaps they will get some warning of the impending attack and be more alert, or perhaps the ominous signal will send a shiver of fright through the defenders.
Here's a thing to try with an adventuring party. When they're camping overnight in the wilderness, have them woken by (or the person on watch hear) the sound of a horn call. Maybe it's off in the distance. Maybe it's quite close by, just off a few tens of metres in the thick forest. The adventurers should expect some sort of imminent attack. And then...
Do this three or four nights in a row for maximum paranoia.
R2-D2: Awesome. An all-in battle.
Chewbacca: The outcome at this point is not at all certain.
Logray: I've run the high speed market simulations. This will offset the contraction of the market in the third quarter by increasing interest rates by 25 basis points.
Logray: Sound the Horn of Plenty!!
Han: Hmm. This might make it harder to steal a ship. Looks like my plan didn't quite work perfectly this time.
Leia: That's a first.
Chewbacca: What, his plan not working perfectly?
Leia: No, him admitting it.