Diaz Lan is a member of the Lan Rogue Trader dynasty, which sent many vessels through the Jericho-Maw Warp Gate with the Achilus Crusade.
A far better diplomat than the crusade's busy commanding Lord Militant Solomon Tetrarchus, Diaz Lan was nominated for the position of crusade liaison, and is often the voice of requests from the crusade high command and lord militant himself to other factions within the Jericho Reach.
Serving as the crusade's errand boy, however, is far from the ambitious Diaz Lan's first choice of occupation. An explorer by heart, he has had to content that wanderlust for many solar decades by gathering the second-hand tales and trophies of others.
Aboard his light cruiser, the Horizon's Pride, the small museum in his library is but the tip of an iceberg. He has secreted away dozens of artefacts, weapons, and documents that would raise many eyebrows if they came to light.
While Lan's status as a Rogue Trader affords him a great deal of leeway in his possessions and interests, the crusade's forces contain enough steadfast Puritans that discretion is in his best interests.
Lan is a scholar and collector, not a sympathiser with the alien's cause. Even if he could power his Helix Gauntlet, he has no more intention of wielding it than he does of converting to the T'au Empire.
To those who make a good impression on the Rogue Trader, he launches an exploratory philosophical conversation about alien lore to see how much it is safe to reveal. And to those who win his trust, Lan proves to be a highly knowledgeable resource about the secrets of the Jericho Reach.
Diplomatic Mission to Aurum
Due to the world's critical importance to the Achilus Crusade as a food-supplier, a source of additional reinforcements, and as the source of the unique Decavane Crystals, the mission was deemed critical by the crusade's High Command.
Unfortunately, the Rogue Trader made little success, with the Aurans refusing to honor future negotiations with him on the grounds of Lan being "not a Man."
Indeed, Lan himself thought it ironic how the Adeptus Astartes were the last hope for peace on Aurum. For in truth, the Lord Militant Tetrarchus had suspicions of an alien landing planet-side.
The need for peace and the planet's resources demanded both compliance from the natives, and the neutralizing of any xenos threat.
- Deathwatch: The Emperor Protects (RPG), pp. 8, 11, 13-14