Its getting to the end of my game and need some ideas for a final fight / boss. Basicly the short version of the story goes like this.

An imperial inquisitor, Inquisitor Taldorus, has spent the last 10 years gathering pieces of chaos artifact in a effort to summon a slanesh theme demon. He aquired these items and the nessasary sacrifices by cloking his actions in the name of the emperor. My kill team came into the picture at the end of this quest, aquired the final artifact and , riped a hole in the warp large enough to provide enough warp energy for the ritual. Of course all by accident. My group still has no idea what his true motives are, but over the next 2 sessions they will find out.

Now i want to have a pre boss battle that looks like the final fight between the kill team and Inquisitor Taldorus followed by the actual battle.

specifically I'm looking for an out of the box idea for a demon and a twist during the kill team vs Big T fight, or some other idea.

Also Inquisitor Taldorus has about 500 sisters of battle at his disposal. They also don't know he is a chaos champion.

if you have any questions post them.

What are you planning on doing with the sisters? When/how are they going to figure out that he's totally corrupt? Will they willingly start fighting a bunch of Deathwatch Marines? Also, why 500? That seems like an awful lot for him to have.

Also, depending on how you're running things, I've found many of the end battles are short and brutal due to the high damage output of PC andNPC alike- the key parts of the fight never seem to be the 'boss fight', but the climb to get to the boss, and the feeling when you finish off a recurring villian that kept getting away with stuff.

How big of a demon are you looking for- the Keeper of Secrets can be pretty much whatever you want it to be, just 'invent' the mood that Slaanesh is in at the time. Personally I'd look for it to be a longer, drawn out conflict, where the demon is actually pulling strings during the fight with the Inquisitors and his supporters- have the thing use the actions of the marines being brave and noble and work in ways to twist them around- this can be exceptionally tricky to do well though. You may have to read into your players a bit here- maybe think up situations that you're confidant of how they'd want to react rather than how their Marines would act and tempt the player to do something that helps the chaos god. Give them some subtle suggestions and give them rewards for doing 'bad' stuff. You'll need to be careful not to make them feel cheated, like they did something they didn't want to by accident or they don't feel the situation was too contrived - maybe by coming up with a way that they can redeem themselves.

**Minor Spoiler**

Depending on how well read they are within the genre you could rip off the daemon manifestation at the end of Ravenor. Use the SOBs as defenders of an isolated fortress, have the inquisitor act as a focus for the summoning (doing something suitable depraved) and then when he is killed reveal he was just a puppet at the end of a pseudopod and actually the creature coming into existence is the size of a building not an individual. It should last a good number of rounds being shot at if it is that big and perhaps rather than dealing damage have it corrupt the flesh of one marine per round in a suitably random and chaotic way.

I do agree with the previous poster, there should definitely be lots of temptation when working the way through the fortress.

Again Spoiler

have the sisters of battle met the Deathwatch marines?

in Grey Knights 1 a rogue inquisitor convinces a whole planet that the Grey Knights are one of the legions that turned against the Emperor during the Heresy, Joe public doesn't know any different and the Deathwatch all in their black armour seem pretty evil and scary.

admitedly the sister of battle captain type person (I forget their ranks) sees through it but only when she actually meets one face to face, so you could have some issues in that.

Also If you check Dark Heresy Ascension there is a Lord of Change who, with a few minor tweaks could make a Keeper of Secrets that will make other masters look like a bit of a joke. wouldnt suggest this for a final boss. for that as people have suggested, massive towering superdaemons are the way forward. something between the amoeba at the end of Evolution and the Watcher in the Water from Fellowship of the Ring

My suggestion: The demon's essence is bound to the artifact, so he cannot be banished / killed until the artifact is completely destroyed.

Unfortunately, the pieces of the artifact are spread through the whole building where the fight takes place. So the marines will be chased through the building by the undying demon while searching for the shards of the artifact.

Don't forget to let crawl hordes of minor demons out of nothing while you chase your marines...

Another suggestion: The demon is a corpse-less "ghost" that is able to control a human or even a space marine. He can switch the bodies whenever his current body is injuring another body in melee.

Off course the ghost-demon tries to get one space marines body. He will start his offense with a horde of minor enemies swarming the PCs – one of these hord-creatures will be controlled by the demon, but which one???

He will swap from one horde-member to the next to get closer to the space marines. At last, he will try to get one of their bodies – that's when things get really bad.

What will the players do? They can try to cripple and kill their former fellow in order to stop the demon, but that's not what they will want to. The solution will be: When the body with the demon inside touches the artifact (or a shard of the artifact), then the demon can be banished. So the PCs will have to push their former companion against the artifact without being hurt by him (because this would enable him to swap bodies again). Don't forget funny moments like: (Two space marines in a brawl) "Kill him, he's the demon." "No, he lies, HE's the demon" "Can't you see he's trying to trick you: Believe my, he's the demon" and so on... (Don't tell your players who of them is really the demon, let all of them speak for themselves. Your players won't forget this moment...)

The planet itself is an ice world with an isolatied inquisitorial fortess outside a massive dome where the general population lives. So telagos has the right idea, the SOB's will be defending the fortress. Over the next 2 sessions my group will meet up with a resistance movement, who knows the inquisitors true motives, giving taldorus the evidence he needs to declare my group traitors. Reguardles of the specifics a large battle breaks out between the SOB's and a number of other factions, while the kill team try's to reach the fortress.

I would like to stay away from the giant monster guy, b/c its the obviouse choice. I love the idea of "the man behind the scenes" that Charmander referred to and ill work this into the next two sessions.

The demon itself possing people and mutating them is great, the only problem is that my group will most likely have no problem just flat out killing each other. If anyone has any ideas on how to prevent that it would be great.

One more thing

In the begging of my campain inqusitor taldorus managed to trick a watch captain, Sendroth aka bad ass, causing him to be sucked into the warp. I would like to bring him back with a bang. The bigger and more out there the better. Any ideas??

Bring him back as a demonhost. Extra pathos as the PCs have to kill him.

E=MC² said:

That's easy: You could decide that killing the host will set the ghost-demon free so he can freely decide which body he wants to pick next. The only way to banish the demon is to get its host-body in contact with that artifact.

(Get sure that your players get the clou in early-battle. You could make use of a Psyker who recognizes what really happens and who tells them most of the truth before he gets killed in an ugly way by the demon.)

Final Fantasy Boss works here...

They fight the Inquisitor and some of the Sisters of Battle, it being said that the Kill-Team is corrupt. They kill the Inquisitor and his blood "seals the deal" on the demon, who possess the body and begins the battle anew though killing the Sisters of Battle as well in an orgy of violence until he is stopped by the Kill-Team. Then he leaves the boy at the last moment and with the amount of bloodshed that just happens it manifests as it's true self.

They defeat this demon. End of story.

Not.

It draws power from the artifact and is brought back to materialization and seems to be even stronger. Still, not above the Kill-Team's abilities to destroy him.

End of Story....just kidding.

The demon comes back again from the artifact, even stronger. This time it seems that he cannot be hurt...or at least until the Kill-Team strike out at the artifact and destroy it and scatter it into it's component pieces. The demon shatters and leaves behind...the Inquisitor. He tries to make a quick case that he was switched with a portion of the demon. It either works and all is happily ever after or he gets an end like the Red Skull does in Ultimate comics. Shot in the head.

My personal favourite twist for Chaos (particularly for Slaanesh) is to have it disguise itself as something which comes from the Emperor.

You could have the Daemon manifest as a golden, angelic figure and congratulate the players for having defeated a servant of Chaos, and make them choose whether they trust it or not.