Springing from a nice layout on Developing A World Through Encounter Tables by Welcome to the Deathtrap on how they make encounter tables I wanted to walk through how I made mine for Southern Reaches.
Given the drop-in, player-guided nature of the game, it becomes more important to get a bunch of ideas down in an encounter table so when I come cold to some nook of the campaign later, it is all ready to go. For other campaigns the players are on a more steady trajectory but for more open world, they could go anywhere. I decided to dust-off the unlockable 'hidden depths' style of encounter tables so I could get everything down in one place.
Rather than going with all the options which is good for creeping conditions, a chaos scale or the like, I just picked out 6d4 and 2d12 to give a day/night shift. This boils down 30 options (with some duplicates) to 23 but pulls a bunch of the interesting things off a special list with a sub-1% chance of getting rolled to give more variation and make some of them more likely to turn up, especially at night.
To populate it, following Deathtrap's workflow, first I picked out some known themes I wanted to work in:
Life amidst the ruins
Old insects, once masters here
Active local dwellers
Then I had a rummage through other swamp sources to get some ideas and inspirations:
Swamp Encounter table from DMG 3.5
d20pfsrd.com Swamp Tables
The encounter table format as Deathtraps main article
d100 Food Found Foraging in the Bayou
Three d100 Battlemap Generators
I had a first go with the day table, then the special, then the night table as in Deathtraps article then I blocked them into the hidden depths format with the 'day' elements on 11-19 and the night elements on 2-10 and 20-24. Practically, 2-4 and 22-24 are very low probability so they got the weird special stuff. Should things get very strange out in the swamp - full moons? - I can switch to d4+d20 that will bump the likelihood of the odder things and push down the chance of running into 'normal' encounters - all without having to change the table.
Example elements for day-time are local travellers, extreme weather, transport with guards, refugees trekking, locals at work. For nighttime we get cultists engaging in rituals, goblin markets, undead, smugglers and monsters prowling. For the special list we have doors to the feywild, representatives of the big factions about their business (monster-hunting, ruin-delving). These are not clear-cut day/night activities and I am happy for the day-time elements to still potentially turn up at night.
My plan is to couple this encounter table w/ terrain challenges (from Frog Factory's battlemap generator) to get some extra variation in each case. I often forget to make the terrain part of the challenge and particularly in this swamp I want to make it front of mind. For NPC's encountered I will just use step 2 of my NPC personality creation workflow for background/ancestry. Should this become a talking encounter of any depth I would run the other steps for their personality and goals but given this is a wilderness encounter table, their immediate concerns are likely to be as described by the entry - travellers to complete their journey, smugglers to be left alone, guard patrols to confirm all is as their lord wishes, etc.
To wrap up I had a think through some sub-components - some beasts to go in 'Predator beast', monsters to go with 'Monster prowling', what happens during "extreme weather" etc.
Taking this all for a test by pre-genning some encounters:
First (daytime, 6d4) we get Transport w/ Guards + River obstacle - has people so roll up the ancestry and get a group of Owl-lings
Second (daytime, 6d4) we get Extreme Weather + Natural obstacle (Deep Mud) - self explanatory, heavy rains + mud, a pure terrain hazard.
Third (nighttime, 2d12) we get a refugee camp of Okapi-folk, fleeing north from the guild mines to the south.
Last (nighttime, 2d12) we get some insectkin, long absent from the area, why are they back now - these are a bunch of sluglings.
I am satisfied with this - nightfall does not mean everyone exits the area but a lot more strange things are out and about.