Euron Greyjoy is Lord Balon Greyjoy's younger brother, banished from the Iron Islands for reasons the family kept quiet even from itself and spent the years since crewing the seas east and south of Westeros aboard the Silence — a ship, by every account of those who've served on it, rowed and sailed by a crew that never speaks, their tongues reportedly cut out so that whatever the Crow's Eye has seen and done stays exactly that private.
He returned to the Iron Islands after Balon's death in 300 AC to claim the driftwood crown at the kingsmoot in Old Wyk, arriving late, uninvited by most of the captains present, and leaving with the crown anyway — defeating his niece Asha and brother Victarion in the choosing through a mixture of plunder, promises, and a claim to have sailed further and seen stranger things than any rival captain could match. Chief among those claims: a horn he calls Dragonbinder, which he says can bind dragons to a rider's will, and which he is prepared to test against Daenerys Targaryen's three at whatever cost the horn demands of the one who blows it.
His reign opens with a campaign meant to marry the Iron Islands' fortunes to the dragon queen's whether she consents to it or not, sending his brother Victarion and a fleet east while he himself plots closer to home. What he is capable of beyond ambition and cruelty — the poisoned wine called shade of the evening, the warlocks he is rumored to have consulted in Qarth, the fratricide the smallfolk whisper about more than the Citadel can confirm — remains, as of the latest reports reaching Oldtown, an open and alarming question.