POSS SPOILER
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The merchant who can upgrade the horn is called Maheer (I think) and he is on the raised platform on the west of Walkeen's Promenade, next to the art dealer.
Speak to him and ask him if he sells magical weapons. If you have the horn, this will allow you to speak to him about it. If not, he will tell you that he specialises in musical instruments, especially horns.
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Time is an illusion. Lunchtime doubly so.
Epona of The Laughing Hyenas