What "Dark Bargain" was made by the Cult Mechanicus to repair the Golden Throne?

Question

In Warhammer 40k, the Golden Throne maintains the physical remnants of the Emperor of Mankind and anchors him to the Astronomocron in the bowels of the Imperial Palace on Terra.

Over the millennia, the Golden Thrones arcane mechanisms have become less efficient, requiring more psykers as fuel and the device has become beyond the capabilities of the Mechanicum to repair.

It is mentioned that the Cult Mechanicus sent anexpedition to Commorragh, home city of the Dark Eldar in the Webway, to gain knowledge to repair the device, and a "Dark bargain" was struck.

What is the nature of this "Dark Bargain"?

Answer 1

Currently, there doesn't seem to be an exact answer as to what this "Dark Bargain" consists of but there is a few clues as to what it might consist of.

What the Mechanicus might be searching for:

1. Haemonculi. According to the wiki page on them, these Dark Eldar are masters in dealing with flesh and are capable of regenerating fallen Dark Eldar so long as enough of the body is left. Though obviously extremely risky dealing with the Dark Eldar in such way, with the Golden Throne failing, this might be the only option they know of.

2. Ancient Eldar Technology. It is a known fact that the Eldar were a massive empire at one point that was so "perfected" in technology that they became too debaucherous since the tech did everything for them and that led to their downfall. It is not too unlikely that what they need lies in the remains of the empire, Commorragh, considering how advanced the tech would need to be so that the vast majority of Eldar no longer needed to work. Additionally, the Dark Eldar have access to some ancient Eldar tech that their Craftworld cousins have lost, such as vat-growing clones and some of the advanced weaponry they use.

What the Dark Eldar would want:

1. Slaves. The Dark Eldar thrive off of slaves whether or not the slaves are working or being violently and horribly tortured. The Imperium already kills thousands of psykers a day to keep the Emperor alive, if the throne was no longer needed I'm sure the Imperium wouldn't mind sending those thousands the way of the Eldar.

2. Genes of the Emperor. If the Haemonculi are actually allowed to have access to the Emperor they have access to terrifying potential at their hands. Whether they betray the Imperium and corrupt the Emperor somehow or simply gain the knowledge of what makes the Emperor tick, they might be able to gain some serious power in that regard, depending on what the source of the Emperor's power is. This is unlikely however, as the Dark Eldar avoid use of psykers and don't generally look into

3. Diplomatic Immunity. Saving The Emperor provides hefty leverage and they might be able to use that to essentially be ignored by the Imperium so they may go mostly unrestricted in Imperium space and military operations against the Dark Eldar would likely stop.(I doubt the Imperium would ever fully allow the Dark Eldar to do as they please in Imperium space, and the Dark Eldar don't generally mind violent raids, having built their society around them.)

4. Trade. Simple trade with the vastness of the Imperium could be incedibly useful for the Dark Eldar as they suddenly would have more of anything then they could ever need.

5. Slow/Stop the Dark Gods. Slaanesh is the Arch-Enemy to the Eldar and Dark Eldar alike. Bringing back the largest threat towards the Dark Gods would heavily benefit all Eldar. Additionally, the failure of the Astronomican would open a new Eye of Terror and drastically increase the power of the Chaos Gods.

Whatever their reasons, the Dark Eldar have much to gain from this and little to lose.

Note: I'm putting this as a community wiki as I'm sure many others know much more about the Dark Eldar and everything else Warhammer 40k related to improve this.