The Balmer were an offshoot of the Aldmer, after they were corrupted by Molag Bal, their newfound god. They attacked the other Aldmer, and began to raze Aldmeris, killing every man, woman, and child in sight. In retaliation, a group of mages came together, and, with the help of Auri-El, cursed the Balmer, transforming them.
The Balmer were driven out of Aldmeris, where they settled in Black Marsh, killing the Ayleids already settled there. They enslaved the Argonians and the Lilmothiit, making them build large cities of black stone. They then began to invade Elsweyr, and the southern parts of Morrowind, (then called Dwemereth).
However, soon the Chimer came. Their prophet, Veloth, had moved on to Morrowind, but the teachings lived on with the Chimer still in Black Marsh. They invoked Azura, and attacked the Balmer, driving them back into the marshes.
With the newfound help of the Chimer, the Lilmothiit revolted, murdering their Elven Masters. With their Empire destroyed, the remaining Balmer fled, hiding in the deep places of the world, or setting sail to the south, attacking parts of Pyandonea, where the greatest gathering of Balmer are today, at constant war with the Maormer.
The latest known record of anyone in Tamriel meeting a Balmer was in 4E 166, when a group of Thalmor ventured into Blackreach, to find a Balmer Mage, who enslaved the Falmer inside. The Thalmor attacked, murdering the Warlord and freeing the Falmer.
The war between the Maormer and the Balmer is ongoing, and the Death Elves have taken much of the eastern side of the Sea Jungle.
Balmer are described as 'evil-looking' versions of the Altmer.
They can live for up to 300 years, like any normal race of mer.
However, their distinguishing feature is their grey skin, and black eyes. Their appearance frightens most people, and some have gone as far as to proclaim them, 'Hideous.'
Some accounts say they have twisted, deformed bodies, but others declare that they have the same body shape as the other Mer.They are the same height as the Altmer.
Balmer doctrine states that they must rid Nirn of every man, mer, and beast that opposes them.