What caused the Mourning? I already know what did in my current campaign, I want to hear your theories.

Also, if you have any ideas of strange effects or areas in the Mournland that could exist I'd also like to hear your ideas and experiences in Eberron games.

I think it was a semi-magical weapon of mass destruction developed and used in total secrecy by high-level authorities within the four other governments involved in the Last War, all of whom wanted to eliminate Cyre.

My working theory for a tentative future campaign I'll run is that the Mournlands are actually a new, immense manifest zone of a recently discovered 14th plane, and that the high concentration of magic weaponry may or may not have attracted it in closer conjunction with Ebberon's material plane. 

 Mordenkainen's has some really good Mournland monster options, especially the Corpse Flower or Vampiric Mist.

In my campaign, the Dreaming Dark was involved in the Mourning. They are involved with the Lord of Blades and are actively trying to create Warforged of their own that have a connection to Dal Quor.

Quote from Thomasatutt >>

 Mordenkainen's has some really good Mournland monster options, especially the Corpse Flower or Vampiric Mist.

In my campaign, the Dreaming Dark was involved in the Mourning. They are involved with the Lord of Blades and are actively trying to create Warforged of their own that have a connection to Dal Quor.

 Ha, funny I actually have canon in my games be slightly like this.

My idea for the origins of the Lord of Blades is that a handful of members of House Cannith in Cyre knew about the Mourning before it happened (I'll explain in a later comment), and they developed special warforged to be used as bodies for their own consciousnesses. One of the players in my Eberron campaign is playing as Lucky Laughter, a warforged battlesmith artificer with an aberrant dragonmark, and he got the aberrant dragonmark from the Mourning transferring and warping the dragonmark of the House Cannith Battle Smith Artificer Gerrith d'Cannith, and infusing it on Lucky's body.

The mind of this powerful artificer was partially transported to Lucky's body, but the Mourning caused the mind of Gerrith to not awaken until Lucky was rebooted, which includes:

• Sleep, which warforged can't do (but Lucky will do if he is rebooted)
• Die and is resurrected
• Unleashes his aberrant dragonmark (which basically means using its power to hurt himself, and even more his enemies, kind of like a bad divine smite)
• Has a powerful Cannith artificer complete Lucky

Once one of these 4 happens to Lucky, Gerrith d'Cannith will awaken and attempt to take control of Lucky's body. There will be a series of mental competitions to see who gains control, with the other lurking in the back part of the mind. Lucky will then sleep, and dream, connecting to the Dal Quor, and will like eating and drinking, but not need it.

In my games the Lord of Blades is inhabited by the mind of the mad archivist artificer Aaren d'Cannith, and has no warforged mind. Aaren d'Cannith believes that warforged are perfect beings, and wishes to claim a country (the mournland) for himself, and wishes to dominate all of Khorvaire. 

Just some of my rulings, I'd like to hear more of yours. 

Here's how I have it in my head when I run Eberron:

What caused the Mourning?

No one, and I do mean NO ONE, knows.

Many, many people are interested in finding out. Expeditions into the Mournland are always in the offing.

But here's the thing: Finding out what actually happened would be a BAD thing.

The Mourning lead to the end of the Last War precisely because no one knew what happened and all the nations of Khorvaire backed down and began peace talks because they didn't know if something similar would happen to them. Cyre was a lost nation and no other nation wanted to be lost as well.

BUT! If someone DID find out what actually happened and that information got out, the nations would know what to avoid and there would be a VERY strong chance that war would break out once again.

So, in MY Eberron, expeditions to the Mournland are carefully monitored by organizations like The Thirteen, The Trust, etc. to insure that no actual answers are ever found. Those who do think they've found an answer would be in grave danger. They also spread many, many rumors with false answers so that if a real answer were found, it would be rejected by the populace as just another hoax.

Quote from TheCubester >>

Here's how I have it in my head when I run Eberron:

What caused the Mourning?

No one, and I do mean NO ONE, knows.

Many, many people are interested in finding out. Expeditions into the Mournland are always in the offing.

But here's the thing: Finding out what actually happened would be a BAD thing.

The Mourning lead to the end of the Last War precisely because no one knew what happened and all the nations of Khorvaire backed down and began peace talks because they didn't know if something similar would happen to them. Cyre was a lost nation and no other nation wanted to be lost as well.

BUT! If someone DID find out what actually happened and that information got out, the nations would know what to avoid and there would be a VERY strong chance that war would break out once again.

So, in MY Eberron, expeditions to the Mournland are carefully monitored by organizations like The Thirteen, The Trust, etc. to insure that no actual answers are ever found. Those who do think they've found an answer would be in grave danger. They also spread many, many rumors with false answers so that if a real answer were found, it would be rejected by the populace as just another hoax.

 That's an idea I might use in my next Eberron campagin, after my characters learn that knowing what caused the Mourning is a bad thing, even more than it is a good thing.

Quote from TheCubester >>

Here's how I have it in my head when I run Eberron:

What caused the Mourning?

No one, and I do mean NO ONE, knows.

Many, many people are interested in finding out. Expeditions into the Mournland are always in the offing.

But here's the thing: Finding out what actually happened would be a BAD thing.

The Mourning lead to the end of the Last War precisely because no one knew what happened and all the nations of Khorvaire backed down and began peace talks because they didn't know if something similar would happen to them. Cyre was a lost nation and no other nation wanted to be lost as well.

BUT! If someone DID find out what actually happened and that information got out, the nations would know what to avoid and there would be a VERY strong chance that war would break out once again.

So, in MY Eberron, expeditions to the Mournland are carefully monitored by organizations like The Thirteen, The Trust, etc. to insure that no actual answers are ever found. Those who do think they've found an answer would be in grave danger. They also spread many, many rumors with false answers so that if a real answer were found, it would be rejected by the populace as just another hoax.

 The problem with organizations keeping people from finding out the truth to prevent the war from breaking out anew though, is that there are still plenty of parties on all sides of the conflict that would want the war to continue, and the only reason they don't is out of fear. The treaty of Thronehold is so recent that many of the tensions from the war are still fresh, many combatants convinced that their side would ultimately have won (and still could), nations convicted of the rightness of their cause over those they feel had wronged them. For that reason, I don't think any organization representing all the powers of Khovaire could exist with that aim. 

That's why I personally considered it important to have an explanation for the Mourning stashed away. It's the question on everybody's lips, and sooner or later a campaign is bound to touch on it. In that case, more interestingly than just telling your players "nobody knows," you can present your players with an interesting moral dilemma upon finding out your version of what happened: if the war didn't cause the Mourning, do they conceal that knowledge to keep the tenuous Thronehold peace alive, allowing the world to live on albeit in fear? Or do they trust in truth itself and make it known so that the world can, for good or I'll, finally move on?

The Mourning is part literary device and DM tool in that regard. Now that I type this actually, the Mourning sounds more and more like the ending to Watchmen, and I bet you could come up with an explanation for it much the same where a person or persons destroyed an entire nation to trick the war into stopping. But again, like in Watchmen, what would you do when reaching the end of the conspiracy? Ensure the safety of the world by staying silent? Never compromise even in the face of armageddon? Presents a lot of cool player choices. 

My current explanation for the Mournland is that Lady Illmarrow, also known as Lady Erandis Vol, who is endeavoring to become the Queen of Death, discovered that Rak Tulkesh was gaining more and more power from the Last War, and was going to continue gaining power until he had sufficient power to break free of his prison, and destroy all creatures on the Material Plane. Erandis could not allow this to happen before she became the Mistress of Death, so she, along with a group of the Order of the Emerald Claw's most powerful priests and necromancers enacted a ritual with an Eldritch Machine known as the Death Spire, during the Ritual of the Mourning (the name spread through a spy in the Korranberg Chronicles) where they sacrificed many of their necromancer's souls to create the area known as the Mournland, to kill everyone in the nation of Cyre to put a pause on the Last War.

This is how the Mourning happened in my current campaign, any questions or plot holes? Any suggestions to make this better? I have more info on how this will play out, and need feedback to make the idea better.

The Kalashtar did it.

In my Eberron the Dreaming Dark incited the Last War, they poisoned the minds of the last king and his heirs to sow chaos. Their plan was to have their Inspired swoop in and bring stability, just like they did in Sarlona.

The Kalashtar have clandestinely been trying to do what they can to create peace, but they have very limited resources. Eventually they hear that the Leader of House Cannith is possed by a Quori, and he has created a Quori Monolith in Cyre. A team of the most powerful Psions the Kalashtar have is sent to deal with this new threat and destroy the monolith. The Mourning is the result of the Psionics used to destroy the monolith. The psionic explosion brought a part of Dal Quor into Eberron, basically Cyre has become a giant manifest zone.

Why does it only affect Cyre? First of all, those bodies that refuse to decompose? They aren't dead, they are in stasis, but they are still dreaming. The horrors of the Mournlands are their nightmares. The reason it fits into the border of Cyre is because the Mourn Lands ARE the nightmare of the Queen of Cyre.

Maybe there are areas of clearness within The Mournland, and the fog has a similar effect as the fog from Curse of Strahd. I am away from my book at the moment, but I believe it involves CON saves vs exhaustion if they spend x amount of time within the fog.

I really like these ideas. Mine are similar to what is posted below in "The Khalashtar did it" post -- the Mournland is a Dal Quor manifest zone, although in mine it was created accidentally by the Inspired agents working within House Cannith. In my campaign, the Sovereign of Blades has stepped back from their campaign of violence, but the Dreaming Dark wants warforged to be perceived as a threat. Inspired Agents have radicalized some followers of the Lord of Blades and encouraged them to join the Cults of the Dragon Below. 

Quote from Whiskeyjack8044 >>

The Kalashtar did it.

In my Eberron the Dreaming Dark incited the Last War, they poisoned the minds of the last king and his heirs to sow chaos. Their plan was to have their Inspired swoop in and bring stability, just like they did in Sarlona.

The Kalashtar have clandestinely been trying to do what they can to create peace, but they have very limited resources. Eventually they hear that the Leader of House Cannith is possed by a Quori, and he has created a Quori Monolith in Cyre. A team of the most powerful Psions the Kalashtar have is sent to deal with this new threat and destroy the monolith. The Mourning is the result of the Psionics used to destroy the monolith. The psionic explosion brought a part of Dal Quor into Eberron, basically Cyre has become a giant manifest zone.

Why does it only affect Cyre? First of all, those bodies that refuse to decompose? They aren't dead, they are in stasis, but they are still dreaming. The horrors of the Mournlands are their nightmares. The reason it fits into the border of Cyre is because the Mourn Lands ARE the nightmare of the Queen of Cyre.

 That's a cool idea, it explains why warforged weren't affected because they don't sleep or dream, but I would think that this also didn't affect Kalashtar because they don't dream either, and this would kind of be a dead give away to the causation of the Mourning, related to Dal Quor in some way. I do think the bodies are dead, maybe their souls/minds are just trapped in Dal Quor, so they awaken from death if resurrected with strange dreams from when they were dead.

Thanks! I want to run a campaign where it happens on a smaller scale in Sharn. It kills about 50% of the city population and creats a hurricane like storm (called the Dirge due to the eerie sounds it makes) around the city that kills any none Warforged who enter it. The survivors are trapped in the ruins of the city fighting over shelter and food. A weekly caravan of Warforged come from the storm with food.

The Kalashtar know what has caused it but they can't admit it and they don't know how to solve it.

Maybe that doesn't work as elegantly as my explanation for the mourn lands, but I think its cool.

Originally I wanted only the queen to be dreaming and the rest to be dead, but I can't think of a reason why that would be. Maybe in your Ebberon Kalashtar AREN'T affected, it becomes a tantalizing clue that you're never actually obligated to explain.

Also, I can't decide if I want the "souls" of the Warforged to be Quori spirits or not. Need to read up on more lore to see if it makes sense.

Quote from Whiskeyjack8044 >>

Thanks! I want to run a campaign where it happens on a smaller scale in Sharn. It kills about 50% of the city population and creats a hurricane like storm (called the Dirge due to the eerie sounds it makes) around the city that kills any none Warforged who enter it. The survivors are trapped in the ruins of the city fighting over shelter and food. A weekly caravan of Warforged come from the storm with food.

The Kalashtar know what has caused it but they can't admit it and they don't know how to solve it.

Maybe that doesn't work as elegantly as my explanation for the mourn lands, but I think its cool.

Originally I wanted only the queen to be dreaming and the rest to be dead, but I can't think of a reason why that would be. Maybe in your Ebberon Kalashtar AREN'T affected, it becomes a tantalizing clue that you're never actually obligated to explain.

Also, I can't decide if I want the "souls" of the Warforged to be Quori spirits or not. Need to read up on more lore to see if it makes sense.

 The Sharn mini version would be strange, who'd be causing it? This could be a cool plot hook, but it would make things difficult. The Skycoach drivers that are affected might not be dead, just dreaming, but their skycoaches will crash and then they're dead.

I would have my Kalashtar likely still not be affected in my campaign if I were to do this.

Warforged souls being quori spirits? That would be interesting, I might steal this. The Creation Forges that make these souls would have to access Dal Quor, or just kill Kalashtar. You'd then have to trap the quori spirits in the Creation Forge, which would then wipe the memories of the Quori, and then the Creation Forge would have to siphon a soul into the body of a Warforged, where it would then be trapped. Then, after death, warforged wouldn't actually die, they'd just be dreaming as they return to Dal Quor. This is a very good idea.

I stole from L.E. Modesitt Jr for mine (specifically The Order War).

Cannith power was rising and starting to tip the balance between the Dragonmark and the crowns.  A group of heroes from Aundair/Eldeen set out to fix it.  Their Artificer had created a series of lenses and dragonshards that could draw in the entirety of the day's light and focus it on a point.  The druid,t paladin and rogue took on the Cannith Enclave's forces and protected the Artificer.

The paladin fell but they did so at great cost and it was too late to stop the weapon.  The light blade knifed through earth, melting stone buildings like candles until it struck something unexpected.  To this day no one has found exactly what Cannith experiment or contraption exploded but the smoke never cleared, at least not from the borders of Cyre.

No one is quite sure what happened to that group of adventurers but the Royal Eyes refuse to acknowledge they ever existed.  Eldeen is silent as well but at least two individuals matching descriptions of the Artificer and Druid have been spotted in the company of Ollean.

This was a Pre end of the War campaign I ran back in 3.5.  I've kept it as backstory in my 5e games.

I am planning on introducing the cause of the mourning into my campaign over the course of a few smaller adventures leading up to a larger campaign.

Basically, the mourning was caused by a device from another plane of existence. The device in question would appear as a childs toy, or puzzlebox.
Once activated, the device would open a gate or portal to the plane of existence where it originated and a silent countdown would begin.   The players would then have to navigate the world of the puzzlebox and defeat or outsmart its inhabitants (who may or may not be aggressive depending on the circumstances).  If the puzzle device/toy is not solved within the time limit it begins to merge its reality with that of Ebberon, essentially causing another Mourning incident.  Now, throughout the adventure the players will eventually discover that this device is only one of an unknown number of other devices that were created.

All of them slightly different than the last and with different methods to solve the puzzle and end the mass destruction.  Thus creating an unknown level of danger within the lands, where conspiracy theories and fear of other nations run rampant.  In the end, it could be that it was all a very destructive child's toy from another plane of existence, brought into the realm long ago and discarded and forgotten about or sold to a collector of oddities and unidentifiable magical items (a curio or occult shop maybe?).   Will the players figure out the devices purpose? or will they unintentionally spread misinformation and begin a new conflict between nations who are steeped in cold war?  Could be many outcomes and I am looking forward to seeing how this story pans out.   I have yet to write all the details but I am going to have to tie in the different nation's interests with regards to a cold war and the fact that anything the players might discover could lead to an international incident, shattering the tentative peace that seems to be upheld for the moment.