Jarrion, Lucretcia, Eleanor and Ezekiel, if you are reading this stop. Because major spoilers.
So im running what started out as the Lure of the Expanse campaign. The players are going towards the endgame of the campaign. A vast sphere in the void created by the Iron Men and Golden Men before the birth of the imperium. (Thus the name Dread Pearl, the world in the original adventure is just the key to gaining access to it.) The sphere is a matrioshka brain constructed around a shard made by the ancients. This shard was created to serve as sort of a safehouse for their knowledge after the war against the necrons went bad.
Then the Iron Men and Golden Men from before the imperium found it and created a giant construct around it. Eventually the Iron Men rebelled and got cast out after a war inside the sphere. Eventually they became the Yu'Vath. Then many years later the Yu'Vath tried to achieve transcendence, to be pure beings living within the warp in the same vein as the Old Ones and Eldar gods had. This failed because the Eldar made the Imperium intevene a few centuries ago during the crusade that founded the Calixis sector.
So now im in the need for ideas for encounters
When the players arrive the orks will be attacking the outside of the sphere and be busy being eaten by nanoswarms and their ships cut in two by the weapons of the sphere.
Meanwhile Eldar will be chasing after the players in an attempt to kill them. As will the rest of the rivals from the Lure of the Expanse.
Lastly there is the spheres own defence systems. Normally it would instantly kill them but by this time the players have deactivated most of the defence systems to gain entry. Still lots of very very weird high tech, warp tech etc will be there and attack them. Constructs of the Yu'Vath will also be attacking alongside the orks.
Lastly what kind of reward should I give my players? It's gotta be epic something like a functioning STC. An idea of what happened to the lost space marine legions etc.