wow that’s weird ...
how did I get it into my head he brought the horn to the kings moot and had it blown when he put forward his claim to the throne.
Is that something I read? lol
Ah found it !
quote “A Feast for Crows
Euron Greyjoy brings what he claims to be a dragon horn he found amongst the smoking ruins of Valyria to the Kingsmoot when the ironborn are electing a new king. The horn’s noise silenced all and ended the possible fight between the supporters of Victarion and Asha Greyjoy.
http://awoiaf.westeros.org/index.php/Dragon_horn