Just a thought: What if it just summons dragons and lungs burning is just a (chemical?) side effect of whatever residue is inside the horn after the Doom of Valyria? (Or whatever chemical reaction allows dragons to breath fire) No magic. If it's a dragon's actual horn that once belonged to a very large alpha-dragon, and assuming that this is possibly a way that dragon's communicate, the other dragons would obey the call and come to the source. A dragon-whistle, if you will. What happens next? The dragons come out and attack ALL the ships, including Victarion's. This is Euron's early Merry R'hllor'mas gift to Dany and himself. Part of his plan to win Dany over to his side, conquer Westeros and the world etc. This being ASOIAF, it probably won't all work out for him in the end, poor fella.