Eldorath Starbane
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.
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 an uncharacteristically (for a Farseer) careless rune reading, he foresaw 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). 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 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 but then sometime later he was defeated again by the Necron lord Anrakyr in the Carnac Campagin.
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). 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.
Older Than You Think
However, Eldorath Starbane was actually introduced in a story from the 3rd Edition Necron codex.
In the story Eldorath was from Craftworld Alaitoc, which was heading towards the Imperium's training facility for Culexus assassins on his orders. He was doing this because a Culexus assassin killed his sister (who was also a Farseer), and the Eldar consider dying at the hands of one to be a horrific fate (as it resulted in complete destruction of the soul; VS being consumed by Slannesh... tough call). Once within striking distance, Eldorath was about to give the order for Alaitoc's forces to destroy it. However, though eager to avenge his sister, Eldorath remembered his Farseer training and read the runes to see the future consequences of his descision; a good move, as every alternate future where Culexus temple is destroyed showed Alaitoc getting practically destroyed as well by an unidentified enemy (possibly Necrons or the Imperium's forces). Eldorath is torn, but swallows his pride before calling off the attack. Though he loved his sister, the price of vengeance was too high; the devastation of Alaitoc and its people. As Alaitoc retreats, the story concludes with a bitter Eldorath deducing that the Necrons and/or the C'tan put the pariah gene in humanity, likening it to sowing crops, and are preparing to harvest them.
Matt Ward most likely took elements of this story when he wrote the 5th Edition codex, but as usual he changed several things, such as Eldorath's competentency. Whether you prefer the 3rd Ed or 5th Ed version, Eldorath is an enemy of the necrons and his story can be taken in whatever direction players desire in their battles.