Creating a Character

Races

Given the Neath's tendency to attract and... collect... cities, a large variety of races make their homes about the Undersea (both native and otherwise). Generally speaking, the respect afforded a particular race varies depending on their location and affiliation. A human in Fallen London is quite unremarkable, for example, but a human in Varchas would likely be an outcast. The unquestionable exception to this is Shapelings, who are universally hated out of principle. Further information regarding the various races of the Neath can be found here, to be consulted if you have questions.
Below is a list of the most common races in the Neath, though not comprehensive. I have included die values for them if you would like to select one randomly. Going weird here is fine, although expect to have some strings attached. Any races not listed here will be reviewed on a per request basis.

Based off of a d20:

Human: 1-5

Half-Elf: 6

Elf: 7

Dragonborn: 8

Dwarf: 9-11

Tielfing: 12

Halfling: 13

Vedalken: 14-15

Tiger: 16

Clay Man: 17-18

Rattus Faber: 19

Shapeling: 20

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Feats

Feats may be taken beginning at level 4 in lieu of the regular Ability Score Improvement. If you need to apply an Ability Score Improvement to follow the associated multiclassing requirements, you may be out of luck until level 8 or 12 or etc... Other than Mobile, all feats in the Player's Handbook should work fine. Racial feats from Xanathar's Guide, on the other hand, are subject to "retexturing" to fit this world. This is a secondary resource for feats, subject to wording in the actual player handbooks.

Backgrounds

All on the table. I will encourage creativity here, as "Sailor" is probably applicable to everyone and their brother in this setting. If you land on some really obscure one that was originally designed for a different setting, let me know and we'll work to tweak it. This is a good resource for backgrounds.

Classes

I've compiled a few that I think are interesting below, but this is up for discussion if you find another cool class/subclass you'd like to try. I reserve the right to tweak any and all homebrew on the fly if needed. This setting lends itself to "mag-tech", or "magic technology", so some classes that normally might be off the table could work. Still no guns though. Please note that due to the multiclassing system mentioned above, there are level restrictions of heavy spellcasting classes. If there's anything not on these two lists that you want to use, please contact me directly with the material and I will take a look at it.
DIVINE INFLUENCE
For those classes in which gods play a role, please turn your attention here for further information. In this setting, power can be derived simply by devoting oneself to the worship of a particular deity. Due to the isolated nature of the Neath in general, however, worship isn't guaranteed to be a two way street: most gods care little about the happenings in this dark microcosm of existence, and some devotees go their entire lives without hearing a voice answer their prayers.

Fighter
Warlord^link
Arcane Archer (XGE)
Cavalier (XGE)
Samurai (XGE)
Banneret (SCAG)
Combat Medic (CMH
Crusader (CMH)
Gladiator (CMH)
Shieldbearer (MHH)
Sword Dancer (MHH)
Witch Hunter (HotO)

Warlock
Lurker in the Deep^link
Fungal Lord^link
Umbral Lurker^link
The Kraken^link
Fathom Horror (tweaked)^link
God-King (tweaked)^link
The Celestial (XGE)
The Hexblade (XGE)
The Undying (SCAG)
The Genie Lord (MHH)
The Light Eater (MHH)
The Cosmic Machine (CAH)
The Crone (CAH)
The Great Turtle (HotO)
The Great Wyrm (HotO)

Sorcerer
Aberrant Mind^link
Imperial Birthright^link
Storm Sorcery^link
Infernal Heritage^link
Ysgardian Ancestry^link
Divine Soul (XGE)
Shadow Magic (XGE)
Giant Soul^link
Runechild (TCS)
Mazeborn (MHH)
Void (HotO)

Rogue
Inquisitive (XGE)
Mastermind (XGE)
Scout (XGE)
Duelist (MHH)
Fixer (MHH)
Zealous Inquisitor (CMH)
Shadow Spy (HotO)
Sawbones ^link
Archaeologist^link

Paladin
Hellsworn^link
Oath of Conquest (XGE)
Oath of Redemption (XGE)
Oath of the Crown (SCAG)
Oath of Radiance (MHH)
Oath of Thunder (MHH)
Oath of the Stalwart Defender (HotO)

Monk
Way of the Drunken Master (XGE)
Way of the Kensei (XGE)
Way of the Sun Soul (XGE)
Way of the Cobalt Soul (TCS)
Way of Ancient Symbols (HotO)
Way of Purity (HotO)
Way of Righteous Fury (HotO)

Ranger (Revised)(^link)
Monster Slayer (XGE)
Apex Predator (CMH)
Bounty Hunter (CMH)
Geomancer (CMH)
Spirit Guardian (CMH)
Ishi (HotO)

Barbarian
Path of the Ancestral Guardian (XGE)
Path of the Zealot (XGE)
Path of the Battlerager (dwarf only) (SCAG)
Path of the Rune Sage (CMH)
Path of the Skinchanger (CMH)
Path of the Sylvan Warden (CMH)
Path of the War Chief (CMH)
Path of the Juggernaut (TCS)
Path of Brawn (HotO)

Bard
College of Glamour (XGE)
College of Swords (XGE)
College of Whispers (XGE)
College of Entropy (MHH)
Greenleaf College (MHH)
College of Fortune (CAH)
College of the Geisha (HotO)

Druid
Circle of Dreams (XGE)
Circle of the Shepherd (XGE)
Circle of Spores (GGR)
Circle of Chaos (HotO)
Circle of Elementalism (HotO)
Circle of the Fey Touched (HotO)
Circle of the Stones (MHH)

Cleric
Forge (XGE)
Grave (XGE)
Order (GGR)
Arcana (SCAG)
Blood (TCS)
Apocalypse (MHH)
hunger (MHH)
Justice (MHH)

Wizard
War Magic (XGE)
Bladesinging (elf only) (XGE)
Angelic Scribe (MHH)
Clockwork Magic (MHH)
Dragon Masks (MHH)
Elven High Magic (elf only) (MHH)
Entropy (MHH)
Illumination (MHH)
Ring Warden (MHH)
Maho (HotO)
The Grey Guild (CAH)