I just had an idea for a setting. Instead of them being sent by an Inquisitor, instead, they all simply wake up on the planet, with no knowledge of how they arrived there. The investigation part would come from trying to learn where they are, and what brought them there in the first place? It would also allow a larger variety of beings that could be accepted into this RP. It would also allow for a mystery of what the true enemy could be. Chaos creatures attempting to issolate them? A sick Xenos Experiment? Eldar Trickery? Whatever it is, it will make itself known, given time.