Rumor has spread that an ancient, pre-Imperial Star Map has been located in Vanity's underhive, a planet already in a warzone. Apparently the map indicates the location of an Eldar Exodite world within Imperial space in the Jericho reach. Some puritans already want to destroy the map as heresy but the Lords of Ordo Xeno's believe the map could be true and may confirm their suspicions that the Eldar have technology that mask an entire solar system from perception in the warp.
Proof of this concept could affect strategy across the entire Imperium. Finding this Exodite world, which may even be a Maiden world, would not give the Imperium access to the resources of the Eldar system and enable them to deny it's use to the multitude of Corsairs which constantly plague the Imperial merchant vessels.
However the Biel-Tan craftworld seem to have been made aware of the map and are trying to destroy it themselves to protect the world. The normaly obtuse Eldar keep their thoughts to themselves but in a rare frank exchange with a Farseer the Ordo Xeno's learned that, to this particular group of Eldar at least the Imperium is their archenemy, due in some part to some perceived 'genocide' during the great crusade and afterwards.
Inquisitor Rathamus was sent to locate the map, but encountered some of the Eldar forces. His entire retinue has been destoyed and he is wounded. Infiltrate the lower levels of the hive, rescue the Inquisitor and defend him as he tries to obtain the map. Be alert the Eldar cannot move freely in the hive and they will be using their foul 'magics' to appear human or to not appear at all. In addition, the Chaos forces that are probing the planets defences might move against you once they spot you in the area; there is a an ancient secret human society dedicated defending the Eldar due to pre-Imperial co-operation with the race; there are parts of the Adeptus Terra and even the Inquisition that may not want you to find the map, either due it's apparently heretical origin or just their unwillingness to open up a fourth front against the Eldar; and final watch your footing in the underhive it's dangerous place for even Astartes, who knows what might be lurking down there.