Has there even posted an explanation of "In a Bright Land"? I've read it about three times now, but I always end up getting distracted and confused and I forget where am I in it and did you know that Bruce Willis' character was dead the whole time?
There is. It goes something like this: Rahserat, Angel of Good Intentions, is being used by Esus in a power-play to gain more souls and corrupt the domain of Death, so he is sent to arrange events that keep Laroth (master of Spirit magic - I believe one of Kylorin's students) headed down a dark path.
This results (and this is original canon) in Laroth using his incredible control over Spirit to twist the minds and souls of the dead who share Arawn's Vault with him, building up a massive powerbase under the nose of the God of Death himself, possible because Arawn is a pretty lax God who hates his new precept. Now that the time is ripe, Esus brokers a deal with Ceridwen to get his agent (Rahserat) into Arawn's Vault as well, to court Laroth and gain his support for Esus, allowing an eventual takeover.
Rahserat does his job, but it depresses him, and the mutable nature of the Vault responds to his feelings and fears, creating a second (false) Laroth based off Rahserat's dismal self-image. Gyra the Archangel of Death helps him out and eventually reveals what his subconscious has done, because she knows he never fell, and so he ends up with a majority share in the souls of Patria, siphoning them away from both Laroth and Arawn to his created False Laroth.
Realizing the power in his hands could go to Esus or could be used for good, Gyra decides to trust him (always had decided) and arranges for his escape from the Vault, with all these souls under his command, and asks that he take along her daughter Korinna ("She cannot...grow....here") to Erebus. Rahserat agrees, and promptly gets owned when he tries to leave, discovering that Esus had tricked Ceridwen and she was pretty pissed off, leaving a trap for Rahserat. Esus had likely planned to sneak his angel out when the time was right and avoid this, but Rahserat had left on his own without informing his God...
So Ceridwen stashes him in her God-killing trap meant for the One, possibly as a test, possibly to trap Esus too, possibly because she knew Rahserat never fell and hoped to lure the One out, possibly because she is seriously incredibly insane. The nature of this trap unifies all things with each other, a zone of total change and yet total stasis, where creation is impossible - the idea would be to seal off the One from being able to do anything without granting everyone and everything an equal share of his power. Rahserat goes insane and drifts through various transformations as the souls that are still tithing to him from the False Laroth arrive and begin to do the same, things of death merging with things of life. This is what will later be known as the Bottomless Tomb.
On Erebus, Korinna has been waiting for Rahserat for centuries, endlessly, straight through the Age of Ice, a little crazy at times herself. There is a gate she's supposed to open for him, but since he got waylaid and never arrived, she can only try again each year on the appointed date of his coming. Eventually she breaks the gate and goes nuts for a while, then comes back to herself and begins to repair the gate. Some combination of her more-then-mortal blood and the nature of the artifact cause its nature to slowly shift as she rebuilds it: where once it led to a passage from Arawn's realm, it now began to turn to Rahserat himself, in his present location...and when she finally finished, Rahserat was able to "hear" her through her blood infused into the gateway and thus deduce its existence, now a part of the Bottomless Tomb. Finally with an exit, he breaks through into Erebus, becoming the Risen Emperor.
And after he has built the beginnings of Patria Reborn, realizes the truth that Gyra knew all along: he is still the unfallen Angel of Good Intentions, and has a choice between that, and between falling for Esus to become the Angel of Regret. The idea was that you'd get a special event depending on whether your alignment (under Rahserat) shifted to Good or Evil, but modular alignment doesn't really allow such a dramatic shift without religion at this point, and Rahserat is agnostic.
TL;DR VERSION: The Risen Emperor is an Angel of Esus who gathered the souls of Patria under his banner, then led them out into Erebus. He is not yet a fallen angel, and must choose between remaining true to Esus and tithing these souls to him in furtherance of Armageddon, or upholding his precept and making the lie of Patria Reborn a reality under his righteous leadership.
(but as to what will be in those new unit_combat, and how the tech tree/civics...Etc will change... we have to ... SPECULATE WILDLY 
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