Battleship Double Team

At one point Battlefleet Gothic Armada campaign will give you an option to get a ceasefire with the Eldar. To pull it out, though, you will need to do a couple of missions alongside one of their leaders. Which can be bad - since AI generally tries to leeroy enemy ships with no care for your Nova Cannon scatters and wide-spread torpedo salvos - or good, since Eldrathain rides around on a Voidstalker Class Battleship with Biel-Tan favor (fixing its boarding game), named "Scythe of Isha".
After this however, the Eldar stop attacking imperial worlds, and there will be a chance that they will attack worlds taken by Chaos/Orks (and you will be able to assist them).