Once upon a time, the Devil herself looked around and said: "Sheeeeeeeeit, if we keep eatin everything, there won't be nothin left! Yo, Devil Jr., find someone smarter 'fore we run out." That someone is the Thief. You may know him as Oric the Awful.
Not long after the Thief found the Warrens, a little cherub approached him and told him he'd been chosen to rule over the place... at a price. See, the Thief had to find some way to fuel the entire underworld, as it needed power to keep moving and consuming. He came up with a genius plan: use HIS cohorts as batteries! A tense alliance was formed between the Devils and the humans.
The Thief was trusted with an unbelievably powerful Soul Star. It granted him dominance over the Devil and ALL her sinister siblings. He could manage them, tell them exactly what they needed to do to run the place HIS way, under threat of retaliation. There were three such Soul Stars, but that's another story.
However... after the first run of his 'experiment', the Devil told him: they know what they need to do, meaning the Thief himself was no longer needed. Keeping him alive was a useless power drain for no return. He'd been fired.
The Thief, gazing up at the quick and permanent death between the Devil's teeth, simply said: "No."
With an impossible strike of his spirit hammer, the Devil's arms were sliced off, a wretched scream erupting from her thousand yard throat. The Thief ascended to Level Seven and SLEW the Devil's sibling there. He ascended to Level Six and SLEW the Devil's sibling there. By Level One, all that remained of this once-powerful race was the crying, mutilated Last Devil herself. The Thief offered her a trade: use the soul power my system grants you to do as I please, and I'll keep you alive. Thus the Last Devil watched over (and became the conduit for) ALL souls passing through her once-powerful dungeon.
The Thief took her minions - her cherubs - as tribute. Just as allegedly immortal as their elders, the cherubs helped watch over the dungeon, ensuring all went according to the Thief's plans. None dared to face Oric's Hammer. The end.
You do not know all of them too well. There are changes every time, but the roles stay the same. Your memory is also hazy, but you will say what you know.
The Target: You don't know a lot about him either. He changes the most. You only know the others are supposed to kill him. However, you're starting to think this is not true because he is NEVER dead when he reaches you. Oh well. You don't believe in fate much.
The Thief: You hate this person. He is the one who originally took your Soul Star and imprisoned you here for defying him. You actually aren't sure what you did to anger him so much, but maybe it had to do with killing people? Brutal, selfish murder usually makes people mad.[/center]
YOU are not the thief. The other one is the thief. You remember now. You cannot die here because the thief is going to live forever.
You cannot let the thief live forever.
Yes. The first victim was a sibling. And the next one. And the next one.
Siblings are always the most delicious. Others have not experienced love, joy, loss. They do not taste as good as a sibling with a soul thrice as full.
Okay, here goes.
We know from Cain's Story that the Warrens is supposed to generate Soul Charge/Power in a way that makes it a renewable resource.
Oric was the one who set this up (perhaps) after receiving a request from the Devil/Last Seraph. Now the plan worked, but as we see in the story Oric outlasted his usefulness somehow. This seems to be related to the fact that keeping Oric alive was more costly than necessary. Oric defeats the Devil and slaughters its kin which were supposed to be immortal. He then gave the Devil a option for survival. To use the power generated to do whatever he (Oric) wants.
Now here's the kicker:
See, the Thief had to find some way to fuel the entire underworld, as it needed power to keep moving and consuming.
The Warrens is mobile. So in addition to keeping himself alive he intends to keep it growing, moving, consuming... but why.
Oric the Awesome is a THIEF.
The answer is loot. We found items, locations, and people from other worlds. The Warrens is basically a cross-dimensional pirate ship.
We already have enough proof to point out that the point of the Target is to generate Soul Charge and putting the Target and any allies in mortal danger is the best way to cultivate it. The Warrens is set up both reward and punishment to provide incentive and grow Soul Charge.
Finally, he knows that killing a person doesn't kill them. But it does leave their Soul Star in a vulnerable state like what happened to Riltia. Somehow it was drained of Soul Charge into a negative level by an outside force. Also, most Soul Stars have a way of using Soul Charge as well.
Thus the Last Devil watched over (and became the conduit for) ALL souls passing through her once-powerful dungeon.
The fact that the Last Devil is a conduit for ALL the souls we can guess that there are ways of accessing a Star without the physical Star.
So once the Target is dead, they can harvest Soul Charge from the Astral Influx. The Charge is then used to restart the cycle and keep Oric alive. Then any profit is used to move or grow the Warrens allowing for the collection of more people and loot.