Euron's plan only really makes sense if he is series about marrying Daenerys or using the horn to gain control of a dragon. The Essos trek is too elaborate and wasteful of resources (if neither of those were true) to simply be a way of disposing of Victarion. Somehow Euron expects the scheme to work in his favor and was either counting on poison killing Victarion or Victarion walking into a trap. Moqorro was unexpected and increasing Victarion's chances but that does not guarantee Euron will not be the one to profit. Either their is something about the horn that makes Euron extraordinarily confident about sending it with the Iron Fleet or he is secretly travelling to Essos. Given the claim that "all of Euron's gifts are poisoned," it is obvious that Victarion's interactions with the dusky woman are extremely dangerous. It is even possible that Euron took on her glamored appearance or that he somehow is coming to Essos as well (perhaps explaining the "corpse on the prow of a ship"). If Euron does end up at Mereen, he may have to dipose of Moqorro to avoid having the red priest contaiminating Daenerys's views of himself. Moqorro basically views Victarion as a tool and ally to help Daenerys; even with the horn he is not that dangerous to her. However, Moqorro already has heard enough from Victarion about Euron to be suspicious of the other squid and cannot risk trusting Daenerys's fate to Euron. He may already have put two and two together with the flame visions that showed threats to Daenerys. Another possibility is that Euron will try to lull his enemies into a false sense of security on a massive scale by allowing Daenerys and Victarion to unite together, bidding his time until they reach Westeros. At that point, he would try to assassinate Victarion and capture/enslave Daenerys. However, the chances of success in such a matter would likely be slim and like a pipe-dream. It would be an enormous gamble. Another thing that lends credence to the idea that Euron knows exactly what the deal with the horn is stems from the fact that he was aware precisely of the consequences of having it blown at the kingsmoot; that was not an experiment. His journeys in Asshai and meetings with mages all over the world may have radically increased Euron's level of knowledge about such artifacts. The question is whether events will thwart his plan or whether he will temporarily triumph. There is also the question of wth happened to his eye.