Euron is considered one of the most evil and ruthless characters in the series and is often compared with the likes of Ramsay Bolton and Rorge when it comes to senseless cruelty (much like Ramsay, who is prone to flaying, Euron cuts the tongues of his recruits, likely to protect the secrets of his journeys and powers, yet he also does it to his victims and prisoners for no apparent reason). In contrast to the likes of Ramsay, however, Euron is far more cunning, calculative, ambitious and combat-proven and is capable of being extremely charming and charismatic in given circumstances, which makes him incredibly dangerous. In many ways, he is the feared ruler that Joffrey and Ramsay only wished to be and has both authority and power to back him up in his evil plans.
In the TV series, he serves as a supporting antagonist in Season 6 and the secondary antagonist of Season 7. In the novels, he also serves as one of the major antagonists, and he is the primary antagonist in the storylines of Aeron, Asha and Victarion, and he is a set up future major threat to Daenerys Targaryen in the upcoming sixth novel.
In the TV adaptation Game of Thrones, he's played by Pilou Asbaek.
The K Team Storyline
LOTM: Challenge of the Heroes
Euron will be one the main villains in this story.