The horn blows a second time, and everyone in attendance hopes it is the last sounding of that deep and resonant tone. A third report sounds. What does this mean to the people who hear it?
Ideas and questions:
Perhaps everyone is aware of some great supernatural evil, or mighty dragon from the nearby wilderness who descends on the area, and the third horn blast warns them all to take cover?
Perhaps it is unrelated to a creature, or an evil entity, but a natural phenomenon. It might be a warning about something like an avalanche, or a deadly fog rolling into the area.
It could be a false alarm—if there is a personal element to sounding the horn, there is a chance for a mistake to be made. That could take your story in a different direction, as the person who blows on the horn could be penalized for their indiscretion.
Is this a warning sound? Does it mean that danger is on the way? Or does the horn blast mean something else, like the death of someone important?
Is the third tone the last one that could possibly sound that makes any sense to those in attendance? Or could a fourth, fifth, or additional tones be sounded? What would that mean?
A signal like a horn would only work if someone was able to hear it. Write about a character who was unfortunately unable to hear the horn blasts, and is unaware of what has transpired. They could quickly learn about it from the actions of the others who are in attendance, or they could be oblivious to what has happened, and it could be a central point of your story.