The dragonbinder seems real to me. Dany noted that in old Valyria dragons were controlled by "binding spells and sorcerous horns" while she controlled Drogon through words and a whip. GRRM let us believe that Euron probably got this horn from Qarth warlocks and became master of the horn using his blood. Once the horn is sound, dragons will come to the master not the blower. Euron may monitor Victarion's actions by warging into the dusty woman. If dragons are critical for Euron, then this plan is fairly risky as Victarion is stupid and has a festered wound, he may never get a chance to reach the dragons.
Anyway, Moqorro interfered. Unlike Melisandre, this red priest had very accurate predictions. He powered Victarion and his ironmen through the long voyage, beat the Volantene fleet to reach Meereen first, and joined the battle at the right time. However, there is really no need for Moqorro to continue serving Victarion faithfully after this.
Moqorro was a man of monstrous size, he looked "burned" and "charred" already. Actually there is no better man than himself to sound the horn.
In the Victarion chapter from TWOW, Victarion gathered three thrall in queue to blow the horn. He did not trust Moqorro and would not let him take the horn. When Moqorro offered to bleed him, Victarion says the dusky woman will do it.
Now, this is getting really interesting. What will happen after this? The Yunkai army is smashed and Battle of Meereen looks like over. Volantis has sent a large fleet, Dany's men need to take them over. Victarion's purpose (sending iron fleet and dragonbinder to Dany) is probably fulfilled, and he is likely to die. Moqorro and Euron will have a fight over the horn and dragons. Tyrion is hinted to ride the white dragon, he is around and Moqorro knows him. How will GRRM link all these events together? Any ideas? What will happen when the dragon horn is sound?