So the two players, one a Khorne devotee who crashes his explosive rigged ship into the psyker black ship while it was in low orbit, then escaped via a shuttle, but was shot down by one of the orbital defense platforms, crashing onto the planet.

The other player is a powerful Tzeench Sorcerror who has a small band of a few hundred cultists on the planet, gathering forces to eventually take it over for years. However, the arrival of the Khorne marine changes all that since now the Adeptus Astartes will get involved as there is a chaos marine spotted. Thus, they will need to make their escape. It will be very difficult, but if they fail, Abaddon will have them teleported to the eye of Terror using a ritual of several sorcerrors.

Once there, the Despoiler will command them to participate in the Skull Harvest, as Abaddon knows that the Khorne marine is powerful in fights. (They start with quite a bit of Exp) They will be given a few hundred cultists to be counted as a warband to be able to participate in Huron Blackheart's Skull Harvest. However, I am not sure how to do this. The Khorne marine will likely be participating in the harvest itself as a Champion, but I will also have to find some things for the Sorcerror to do.

And help and comments? (They start at 5k exp)

You could have the sorceror first trying not to draw khornes attention during this ritual, and second try to turn it to some tzeenchian use rather than khorne.

It seems to have changed, since the Sorceror has decided to bind a daemon to an imperial light cruiser instead of a crew and make off with it after shutting down the orbital defenses and sowing chaos with mass suicide bombers.

And what ritual are you talking about? The skull harvest is a gladiatorial arena where the victor slays the rest of the leaders and absorb everybody's warbands into their own, as well as being rewarded by a temporary fleet by Huron Blackheart.

I was using the term ritual descriptively, not meaning it was a ritual rules-wise. Maybe calling it a tradition would have been less confusing.

Awesome use of a concept I really enjoyed the short story the Skull Harvest.

To get them there however I wouldn't recommend using Abaddon the Despoiler as he wouldn't reward failure with a chance to lead your own Black Crusade, more likely just a very painful death or worse. Besides most of the champions who fight in the Skull Harvest do so for the glory of leading their own Black Crusade and are already leaders in their own right.

Since the characters seem to have already escaped you could have the characters come across details of the next tournament along with the rewards of victory at the tournament (that being leading an army comprised of all the defeated warlord's armies with which to lead a mighty black crusade). That should entice them to head there post haste. Perhaps their reputations may even have them being invited to attend.

Once there the Sorcerer could actually be quite useful either manipulating behind the scenes to ensure that the most powerful rivals are drawn to face each other thus weaking the potential challenges that the Khorne Berserker would need to face within the pit. He could also use his powers and other abilities to weaken those that are drawn against the champion of Khorne to increase his odds of winning even further.

Hopefully those ideas will help a little bit.

Good luck sounds like a fun game!