>The Tallarn Desert Raiders continued their attack on various worlds which had turned to Chaos until sometime around the 30th Millennium, when they were recalled. The "Cursus," a Chaos relic of black stone which had led the Iron Warriors to the world was discovered deep under the desert sands. Immediately after finding this relic both Eldar of the Biel-tan craftworld and Iron Warriors spilled from the sky once more, to attempt to claim this relic. After months of fighting, the Eldar and Tallarn Desert Raiders formed an alliance and destroyed the Chaos forces.
>After their combined victory over the Chaos hordes, both races exchanged their promises of friendship, before the Eldar departed in peace. The Tallarn re-entombed the Chaos relic beneath the sulphur sands and turned their backs on it.
http://wh40k.lexicanum.com/wiki/Tallarn
>Once the Eldar had departed in peace, and the people of both races had exchanged their promises of friendship, the Tallarn returned to the Cursus. They found the black stone cold and lifeless once more, just as it was when they had first uncovered it. However, they knew now that the stone was not dead but merely sleeping, awaiting its time again, waiting for the call of its evil masters.
>The Tallarn buried the Cursus beneath the sulphur sands once more and placed within its circle the mysterious devices that the Eldar had given them for that purpose. Then they sealed the surface with plascrete and turned their backs upon it.
Has this bit of fluff ever been mentioned anywhere else besides the 2nd Edition Imperial Guard Codex?
Because based on Lucien Soulban's Tallarn novel, the Tallarn take these kinds of oaths very seriously. I'm not sure about the war-like Biel-tan Eldar, but I'd imagine that if a Tallarn regiment were ever put in a position where they would be pitted against Biel-tan Craftworld Eldar, they may be tempted to look the other way so that they escape, or something.