From A Wiki of Ice and Fire
The Dragon Horn was brought to the Kingsmoot by Euron Greyjoy whent he ironborn were electing a new king. The Horns noise silenced all and ended the possible fight between the supporters of Victarion and Asha Greyjoy. The horns noise according to witnesses sounded like the screaming of a thousand souls and made it seem to the listeners as if there bones were aflame and searing there very flesh from within. Cragorn the man who blew the horn collapsed with blisters on his lips and the tattoo of a bird on his chest was bleeding, he died later and when a maestar cut him open to examine the cause of death it was discovered his lungs were charred black as soot.
"I am Dragonbinder, No Mortal man should sound me and live, Blood for Fire, Fire for Blood"
Moqorro says whoever blows the horn will die but any dragons that hear will obey the Horns master, Moqorro says Victrion must be made the horns master and he must claim the horn with blood.