Origins and relationships
Second Battle of Mag Tuired
When Bres was deposed by his fellow Tuatha de Danaan and learned of his Fomorian heritage, he fled to them and convinced them to attack the Tuatha. Balor was the general of the Fomorian army during the Second Battle of Mag Tuired, and was grievously injured by his grandson. The Fomorians were defeated and forced to retreat to the Dark Realm of Otherworld.
At some point, Balor was imprisoned by his fellow Fomorians led by Elathan (known to Balor as "Goldenskin", seemingly because of his crave for destruction and because he didn't distinguished allies from enemies. His wife Cethlann was given a seat at the Fomorian council out of deference to him.
When the Avengers were brought in the 12th century AD by Amergin Glúingel to battle the Fomor who had invaded Avalon, Cethlann proposed to unleash Balor to deal with them but her proposal was refused by all of the council.
When the battle weighted in favor of the Avengers, Cethlann released her husband who swiftly dealt with Thor and She-Hulk, the strongest Avengers present, by disrupting the spell keeping them in Avalon in that era and returning them to the modern age. The other retreated to evade the monster. Elathan managed to gain the loyalty of Balor, despite Cethlann's protests, leading him to raze Avalon and reach out Earth (but intending to suppress him and Cethlann as soon as the deed would be done).
Arriving at the city, the Fomor were faced by Amergin, who used the Evil Eye to drain Balor's power. Profiting from the brawl (including Elathan attacking Amergin), Balor took the Evil Eye, intending to take his energy back, and was apparently destroyed but his life essence was truly imprisoned within the Evil Eye). The Black Knight then used its powers to seal off the gateway, with only a slight residue of Balor's energy remaining in it, the rest being cast off at the four corners of the Multiverse.
The residue grew over time, until the point Morgan Le Fay and Mordred were able to unleash a reconstructed Balor from it, using him to battle the Black Knight (Sir Percy) and Doctor Strange, then Valkyrie (Brunnhilde possessing Victoria Bentley). Balor's life essence dissipated after the Black Knight destroyed the Evil Eye, and Strange sealed this dispersal by a spell invoking Oshtur.
Powers and Abilities
He seemingly can't open his eyes without firing his blasts, making him functionally blind on the battlefield when not using his powers.