After their Uncle Euron murdered their father and stole the Salt Throne for himself, Yara and Theon fled the Iron Islands with a fleet of their fastest ships. They landed in Meereen and agreed to help Daenerys conquer Westeros, as long as Yara gets to keep the Iron Islands in the end.

This left Euron on the Iron Islands, angry and eager to build new ships and come after his niece and nephew.