I am currently undecided on the system to use for this game. It is clear that whatever system I use, it will need some setting specific modification.
The current candidates, in rough order of likelihood are:
Immortals on Velocipedes is my custom modification of Kids on Bikes, a lightweight narrative based system. One of the greatest strengths, and weaknesses, of the system is its simplicity. It is very quick to understand and start playing, however, almost everything has to be solved by narrative or a single contested roll. There is no real combat system, or special rules for handling various scenarios. This leaves lots of room for narrative control rather than pages and pages of spells and features, but could be underwhelming for those who like detailed tactics and finding clever mechanical builds and exploits.
It's simplicity also makes it easy to create a custom ruleset for virtually any setting. A full breakdown of the new rules, including character creation, can be found here, but here is a quick summary of the changes.
The Age mechanic is replaced by Natures, relating to a character's origin. All characters are assumed to be mature adults unless stated otherwise.
Each Nature has a specific weakness, such as Holy Water or Hellfire making rolls against them stronger.
The tropes and character creation questions are instead based on the world lore.
There is a Miracles mechanic for certain natures, loosely based on the Magic system from Kids on Brooms. Allowing characters to perform impossible feats in certain situations, adding an extra die.
Strengths are renamed Feats. There are also special feats known as Powers, which are only available to those who can perform Miracles.
Adversity Tokens are renamed Destiny, and the GM may award extra Destiny between sessions based on the characters do between them.
A PBTA based system might allow more meaty combat mechanics while still being more lightweight and narrative than DnD. Such a system would require more effort to make and play than Immortals on Velocipedes however.
I am excited about the upcoming MCDM RPG and it might make a decent tactical system. The system however is not only high fantasy focused but unfinished. It is currently difficult to say how easily it could be translated to this world.
It would likely involve creating custom ancestries and possibly limiting the class/subclass options for each.
Dnd is likely not a great fit for this game, it will require a lot of limitations and modifications to make it fit. However, it could work and I am familiar with the system.
I would likely make custom races/lineage, loosely inspired by tieflings and aasimar, with limitations on which classes each can play.
Currently I expect to be the only GM running this world, maybe one day I'll publish whole adventures for other GMs. For now though, this section is primarily for handouts relating to each adventures.