It's been a while, but I've been writing a Player's Guide conversion to Eberron. I've finished part 1 to 4 in its first draft version. I copied and pasted liberally from several Eberron books so I think I would violate all kinds of copyright rules if I would upload them to this thread. I did make significant changes to the conversion from the original post, due to your comments and giving more thought to everything. Long post about the history of the Obs in Eberron coming.
History of the Obscurati
So one of the problems was a history of 500 years of the obscurati and the Mourning being 4 years ago. I couldn't solve this problem, so I changed it. I made Nicodemus a person from the time of the War of the Mark (1500 years ago) having an aberrant mark. For reverence to the war of the mark, see this article by Keith Baker:
sidebar-tarkanan
Before the War of the Mark there was a war of Eladrin fighting against the human start up nations. Kasvarina and her husband fought against Bregor of Wroat (current Breland). Nicodemus and Tarkanan fought against them. Kasvarina was captured and Nicodemus took care of her, because Kasvarina's daughter had an aberrant dragonmark (causing infertility) . When the dragonmarked houses started to aid King Bregor, they ambushed the aberrants. Nicodemus and Tarkanan survived, and the aberrants and eladrin joined sides. The war spread to other parts of Khorvaire, and focused against the dragonmarked houses. During the war, Tarkanan asked Nicodemus to do peace negotiations with the dragonmarked houses. Kasvarina was chosen as the diplomat for the Eladrin. They were taken prisoner, and receive the ritual as described in the adventure path, although the demon is working for Sul Khatesh (overlord of arcane might and mystery). They are taken to Metrol capitol of Cyre and deliver the ritual to the dragonmarked houses. The dragonmarked houses perform the ritual on the daughter of Kasvarina and deliver her through a portal to the final battle of the war in Sharn. She is slain, and not only are most persons with an aberrant dragonmark slain (including Nicodemus, who lives on as a sort of ghost), as well most eladrin become infertile.
This causes the eladrin to retreat to their Fey Spires, and retreating the Fey Spire to Thelanis. The leaders of the aberrants do sacrifice themselves, destroying the city of Sharn, and causing madness and plagues in the area as described in Eberron canon. Kasvarina herself survives, and returns to Thelanis with the other eladrin.
The dragonmarked houses are shaken by the effect of the ritual decide to never use it again, and to rerwrite history to blame the aberrant dragonmarks for everything that happened, and never mention the eladrin in this war. They archive this ritual with house Cannith in Eston.
After approximately 470 years Nicodemus starts a philosophical enclave in Xandrar. The city has always been known for its tiefling population due to the many planar influence from the blackcaps mountains. This is described as the Pala part in the history of the Obs. Kasvarina joins him in the city and the Jierre family are the ruling nobles in the area. His philosophy is for a big part against the at that moment huge influence of the dragonmarked houses. When Galifar I enlists the dragonmarked houses with the first version of the Korth Edicts Deneith and Cannith want to help Galifar conquer the continent but the want to destroy the philosophical heretics in Xandrar. Galifar agrees and ultimately Nicodemus is burned on the pyre. Galifar wins his kingdom with the support of the dragonmarked houses.
A couple of years later the archduke of Xandrar Jierre develops an aberrant dragonmark and together with Kasvarina and Nicodemus they found the Obscurati. If I put it at this point in time, they have almost 1,000 years to grow and be influential in all layers of society. This is the shadow cabinet behind the later foundation of the Aurum. The Aurum is used by them to gain new members. One of the main goals of the Obs is to limit the power of the Dragonmarked Houses, and possibly the draconic prophecy itself.
Enter the Last War. After 100 years of war a big battle is going on in Cyre with armies of three countries making it very difficult for Cyre. The Queen of Cyre has spend enormous amounts of money on House Cannith for Warforged armies, but it still doesn't look good. The head of House Cannith proposes to do a ritual on the Queen to give her the power of all the people living in the country. The ritual is performed in Metrol again, but the queen is immediately afterward murdered by an Obs paid murderer, triggering the Mourning. As such Metrol will take the role of Methia as described in the adventure path. The Mourning caused the Eladrin Fey Spires to reappear on Khovaire and not be able to return to Thelanis. In my Eberron elves are descendants of eladrin, that lost their connection to Thelanis. Just for reference.
So one of the issues I have is the use of the ritual later in the adventure path by multiple persons. The sacrifice of gods is also illogical in the Eberron world, so I changed it. I think this is a good fit. Anyone with a different opinion?
So house Cannith is going to be important. As suggested earlier I'm going to make Pemberton in league with House Cannith South. He is in direct contact with Merrix d'Cannith. One of my player's wanted to play a Cannith marked human. He doesn't know who his real father is, and knows it's been a scandal in Fairhaven. He has been raised in Windshire by his mother and another father. In reality he will be the half-brother of Merrix, which will make part 6 really good.
Fey Titans
The Fey Titans were original champions of Eberron in the Age of Demons. After this war ended their might slowly declined, but stories about them started to spread. As such they gained a bond with the world of Thelanis and became the Fey Titans with manifestations in both worlds. The dreaming, I call it the Feywild to have no confusion with Dal Quor, the plane of dreams, is a layer of Thelanis, which overlaps with Khorvaire, centered on the former area of Aundair. As described in Exploring Eberron archfey are the foundation of Thelanis, the force that sets a story in motion. Archfey that rule a layer are called Anchor Barons. As such, king Thisraldion of the Unseen Court is the Anchor Baron of the Feywild. So ultimately the Unseen Court handles the Fey Titans and the kings and queens of Aundair have a pact with the Unseen Court. Duchess Ethelyn of Windshire is the official emmissary of Aundair to the Unseen Court, and Asrabay Varal is the official emmissary of the Unseen Court to Aundair. Asrabay Varal and a contingent of eladrin warriors helped Aundair during some battles in the Last War, so he is known in Aundair.
The Planes
I'm also taking the advice of the planes made by
@Galandris. Not using the eight aspects as originally described but giving each plane a focus of what it means for Eberron. I've not described what will happen to the world if a current plane is removed. I'm planning to do that in the coming month. The Bleak Gate will be a layer of Dolurrh, which is difficult to access. I did write down in the Player's Guide that planar travel except to Thelanis is extremely difficult, so no summoning or planeshifts in my Eberron.
I'm starting the campaign in one or two months, so fortunately I have some time left. I'm also starting with different adventures because I like starting a campaign a 1st level instead of 3rd level. Those first two levels will be used to become agents of the Royal Eyes of Aundair.