I have searched far and wide for answers, and as nothing substantial has been found I come here asking the big question: what is/could be in the Threnos Zone? I know there's no established cannon; I'd just like some ideas/brainstorming.

We know that:
1. It is a system of 13 dead worlds, with satellites constantly broadcasting Imperial litanies towards it.
2. The system has been declared utterly forbidden by Inquisitorial mandate.
3. A fairly large garrison of Battlefleet Calixis is kept nearby, despite the fact that the system is supposedly "dead".
4. The First Cloister of the Calixian Ordo Malleus is on the moon of a world on the system's outskirts.

My thoughts are that it must be warp/Chaos related somehow, hence the Imperial litany-broadcasting satellites, as well as the Cloister. But beyond that, I'm lost. Could it be a system once (or still) held by the Yu'Vath or their relics, considering that they harnessed the powers of the warp in their foul technologies? Perhaps it is somehow the opening to a way into the Screaming Vortex, perhaps the 13th Station of Passage (it is a system of 13 worlds after all)? The latter seems unlikely, due to the location of the Vortex, that not many Imperials would be likely to know of this, and that I believe it says in the Black Crusade corebook that the 13th is in The Maw.

Ideas?

And another mystery: what is the connection between Threnos (dead word in the Hazeroth Abyss) and the Threnos Zone, given that they're a sub-sector apart?

Great question!

The Lovecraftian in me is bound to come up with some typical answer- the soothing litanies can't help but remind me of the Servitors of the Outer-Gods playing the flute in order to keep Azatoth slumbering in his court.

13, as FFG love to remind us, is also extremely linked to the Haarlock Mythos. Granted, there's been no established connection between that System and the man. I'd need to look into the Mythos of the Drusus Marches themselves, perhaps the big man himself had something to do with the sealing of the planet?

A connection between both Worlds could be physical- an actual portal links the two areas together. But that's too obviously. Maybe that one Planet is a Dream- a reflection of One of the planets in the Threnos Zone. That would be cool: the one planet constantly changes to reflect all thirteen others. Why is it so far? Thats where numerology comes in. Perhaps, when Space-Time is folded, we'll find that the planet is actually at the center of some large diagram made up of the entire set?

I am not sure- I don't think Space-time works that way

I like the idea that there's something slumbering in there, and that the thirteen planets are locks or gates that keep it sealed. Maybe they aren't planet in the sense of the word, but actually alive… "That's no moon!" type thing… that would be cool…

Man, you're good at this!

Perhaps it is the final resting place of the Yu'Vath, despite the fact that we are generally lead to believe that they're somewhere (or their remains are, at least) in the Koronus Expanse? Ooh, perhaps the Bale Childer (related to them somehow, though we're never told if it is a being or a species) is imprisoned there?
Additionally, I've heard it referred to (not by FFG though I think) as a temporal distortion, like a warp rift of sorts. Perhaps it is simply mundane enough as a daemon being held there?

It could be that both of them are connected to it. Haarlock was there a long time ago, so maybe he hid something there, perhaps related to the potential prisoner. Or perhaps Drusus imprisoned who/whatever there using one of Haarlock's toys?

Space-time might not work that way, but Warp-time might.

That is particuarly interesting. Perhaps with the planet Threnos being part of the puzzle?

On the side, I've also considered the fact that this is /somehow/ related to the Tyrant Star, but I think it would tie into the Haarlock Legacy somehow, were that the case. Perhaps it is that, AND a prison? Maybe those things and more?

Just a thought I had reading this thread.

Perhaps it is a prison system for daemons. More than just the slumbering place for one thing there is a faction in the Ordo Malleus wrestling with the fact that even if you "kill" a daemon it is just banished back into the warp and eventually will return again. The 13 planets plus the litanies, probably plus a whole bunch of other things produce a warded zone that is used to seal captured daemons away where they can do no harm. Once in there if they can be kept there they will do no harm, rather than being able to plot and scheme and drift to wherever the warp/realspace boundary is weak.

The planet Thenos could then be a potential escape route for daemons clever enough to use it. Precisely because it is so far away no one has realised that the name of the place itself is dangerous. Just by naming it Threnos the daemons have a link to a location outside their prison.

I would imagine there would have to be a detachment of Grey Knights around to help guard this place just in case along with the Navy.

In my campaign, I use the Threnos Zone as the former capitol system of the Yu'Vath Empire. The thirteen worlds have been bombarded by the Angevin Crusade into lifeless worlds of blackened glass, but there are still untold leagues of tunnels beneath the surfaces filled with undiscovered Yu'Vath artifacts. Since these are so highly prized by the Cold Trade- and so dangerous to the Imperium- the Calixis Sector Deathwatch has established their orbital Watch Fortress in this system, with a network of hundreds of satalites broadcasting Imperial Hymns at the system's edge to hinder navigation (by all unauthorized vessels without the code to temporarily deactivate a satalite) and thwart treasure-seekers looking to loot Yu'Vath artifacts. Even so, mysterious forces prize the warp-tainted technology of the Yu'Vath so highly that they occasionally hire small armies of mercenaries to attempt to plunder the dead worlds; so far, Watch Station Threnos has beaten back all such attempts, but Watch Commander Ramius has the growing suspition that these attacks have simply been probing for weaknesses, in anticipation of a major offensive…

[i run a Dark Heresy campaign as my primary game, but when we can't get enough of our six regular players together, we play Deathwatch (which lends itself better to episodic, combat-focused missions) instead. I don't really care for the Jericho Reach setting of the Deathwatch game, so I set my DW campaign in the Calixis Sector to cut down on the setting details I have to keep track of…]