Eldorath Starbane is an Eldar Farseer of Alaitoc and an arrogant bastard even among the Eldar themselves. This is mostly shown in the huge defeat of his Warhost as a result of the Siege of the Tomb World Somonor sometime in M41 and the string of defeats since.
The battle began in the Eldar's favour, though his prescience was muddled by a combination of Imotekh's tactical skill and Orikan the Diviner's own abilities. Doing a characteristically (for a Farseer) careless rune reading, he foresaw great victory after the retreat of Imotekh the Stormlord and pressed on deeper into the Necron catacombs under the planet's surface (not a very good idea), not realizing it was an obvious trap (or the obvious fact that the runes never said WHOSE victory it would be, and Eldar curb-stomping much weaker Necron army cannot be considered a "great victory", unlike the scenario of Necrons overcoming much stronger Eldar army). After being sealed inside the tombs, with the only way out closed by the Quantum Shield, he was ambushed by Necron Scarabs and then Flayed Ones and finally Immortals who obliterated the grouped Eldar as they were trying to fight off the Flayed ones. Eventually, the only Eldar left standing was Starbane who was obviously thinking one word: OOPS. So then Imotekh had his right hand cut/ripped off as a sign of humiliation and let him get away to his Craftworld. Starbane has pledged vengeance against the Sautekh Dynasty ever since. Good times.
He then went off fighting some Orks in Rynn's world with the Crimson Fists, one of the few times he won, affirming that old maxim that the only time an Eldar can win something is if he's helping a Space Marine (something Games Workshop finally seems to have embraced with the whole alliance with the Imperium thing), but then sometime later he was defeated again by the Necron lord Anrakyr in the Carnac Campaign. Which was a weird event anyway, as the odds of an Exodite World being a Tomb World are...basically zero. Less than zero. Times infinity.
He seems to be something of a whipping boy for Matt Ward. He features in the White Dwarf battle report introducing the Necrons, in an army allied with the Ultramarines (despite the fact that the Imperium are supposed to be Alaitoc's main enemies, but that's Ward for you, however that's more of a "Imperials squatting on Maiden Worlds" thing and less of a "we're trying to destroy each other" thing, they don't particularly hate each other outside of colony squabbles). The Eldar and Imperium barely win, but after the battle Trazyn captures Eldorath and gives him to the Ultramarines as a captive, most likely averting his planned vengeance against the Sautekh dynasty. Although, the Ultramarines being "Honor before Reason" who knows what they actually did with him. Probably gave him a cookie and sent him home or something.