Here is a massively underrated villain from Celtic (let alone, any) mythology. In Celtic mythology, Balor was the one-eyed king of the Fomorians, and the father of Ethniu, who would marry Cian and give birth to Lugh. There are varying sources on how he got his single eye, but the most accepted story is that he was cursed. With his "evil eye", he could VAPORIZE ANYTHING IN HIS SIGHT, and it took nine hooks to lift open his eyelid.

During the last battle of Magh Tuired (between the Tuatha De Danaan and the Fomorians), Balor met his end when his grandson, Lugh (who sided with the Tuatha De Danaan, or the Celtic gods) swung a slingshot through his eye, killing the Fomorian king, and vaporizing a few of his men.

Once again, I'd like to thank Jean Verville for the inspiration, the original picture can be found here: https://www.artstation.com/artwork/rRBYLG