What is the difference between the darkness and the sea?
From a game mechanical perspective it’s simple: is the price of entry a purchasable 1cp (a torch) or a you-have-to-quest-for-it 100gp (a potion of water breathing)? Okay yes the specific penalties are different, don’t be a nitpicker. My point is that darkness is a condition only if you do something wrong, while the sea is a barrier which must always be overcome… or is it?
We don’t think of darkness as a soft fence for exploration because we have fire and light; we do think of water as such a barrier because we can’t breathe underwater. But our characters (exceptional though they may be) can and will surpass every soft fence we create. They’ll overcome physical barriers with water breathing, gaseous form, fly and blink. They’ll overcome informational barriers with tongues, speak with plants, speak with dead and sending. They’ll never even enter a town again, tongue-in-cheek, thanks to the accommodations of leomund’s tiny hut and create food and water. And of course, they’ll drive back the darkness with daylight — after having elven darkvision and the light cantrip since first level, or darkvision since third. And torches. And glowing +1 swords.
So after a certain point, there isn’t a difference, and the sea is just as conquered as the darkness. Humanity spreads. But what did we gain by waiting for 5th level? It means our first level characters can’t adventure with mermaids. And that just plain sucks. Let’s fix it, without invalidating the awesome strides our wizards and clerics and druids will be making.
First, the inhospitable. The 5e DMG puts the planes of fire and frost as, on average, as bad as the deserts on earth: you risk exposure, but not literally-on-fire-every-round temperatures. They have specific rules on exposure, but I like mine better, and the specific differences aren’t important here. The interesting thing about the rules is that in a cold environment, “winter gear” protects you (what is winter gear?), as does shelter and warmth (perhaps a tent and a bonfire?); while in a hot environment, “water to drink” protects you. There are very mundane protective steps you can take in these environments. So reading in between the lines:
Hide armor. (15gp) Hide armor (per the armor table) which is made from a variety of northern furry creatures provides protection from cold environments; saving throws from exposure to cold are made on a per-day basis instead of a per-hour basis. One hour worth of shelter and warmth restarts the clock.
Traveler’s Garb. (1gp) Modeled as robes (cannot be combined with armor), this diaphanous garb protects against heat. Saving throws from exposure to heat are made on a per-day basis instead of a per-hour basis. One hour worth of shelter and cooling restarts the clock.
Fremen Stillsuit. (75 gp) As leather armor that, additionally, protects against heat. Saving throws from exposure to heat are made on a per-week instead of per-hour basis, and you do not need to consume water during this period. Consuming 8 pints of water resets this period.
What did we learn? Much as the price of carrying a torch to enter the darkness is foregoing a second weapon/two handed weapon (… and, to a lesser degree, shield), the price of entering a hot or cold area is that the environment dictates which pants you get to wear. My rules are a little more punitive than the base ones — theirs starts the exposure save DC very low and has furry pants grant complete immunity. But you wouldn’t really know it at the table, and who doesn’t light a campfire each night?
Let’s talk about the wine-dark sea for a moment.
Airy Water Oil. (10gp, 1 lb). A metallic oil which burns with a cool, barely-visible flame (burns as oil but sheds only dim light to the fixture’s bright radius, and no light beyond that radius). Water within its light radius becomes airy water, which can be partially breathed by air breathers. Exposure checks are made on a per-hour instead of per-minute basis, and you cannot lose your breath. The water suppresses flame as normal.
Shots fired! By burning this stuff, air breathers really can treat the sea as though it were the dark, venturing on forays away from their bubble domes. Lighting it is a bit of a pain — might I suggest shocking grasp? Burn it in pipes. Fill a fishbowl with it and put it on your head. It also functions the way torches or lanterns in a dungeon do: lamps every 15 feet keep the tunnels breathable, or perhaps further and holding your breath is expected.
10gp might be too cheap for 6 hours of underwater adventuring, but you can always say the stuff decays quickly, so can only be used a certain travel radius from its manufacture. I imagine this stuff is a hydrolysis-inducing tincture the elves make and burn in mist lamps, which (even on land) hold a special place in their society.
How about humanity’s famously poor night vision, and drow’s famously poor dayvision?
Potion of Darkvision. (50gp, a “PHB-style” common item, like potion of healing). You gain darkvision 60 feet for 1 hour, as per a dwarf, or improve your darkvision radius by 60 feet. While under the effect of this potion, you gain sunlight sensitivity (as a drow).
Potion of Lightvision. (10gp). You lose any sunlight or bright light sensitivity — but not weakness or hypersensitivity — for 24 hours.
That easy — maybe even trivial. Antivenom, which significantly weakens green dragons, serpents, scorpions, wyverns, drow, etc is right there in the book at a similar price point. And that has a strong combat effect — let’s just assume everyone can eat their carrots and move on, since otherwise human rogues have a very difficult time of it.
What about flight? Actually offering full flight really does expand the players’ abilities in ways I’m pretty uncomfortable with, actually, because it has a direct combat application. Be an archer. Be able to levitate. Dictate all terms of engagement. So flight needs to basically be at the plot level: this castle can fly, and goes as the wind will. This dungeon is on an earthmote. That sort of thing.
And then you need to actually get to the castle. You could climb a tall mountain and wait, of course. You could use a catapult and some trigonometry. You could tame a hippogriff. Frankly, all you need to do to put travel-based flight in the hands of players is to put hippogriffs in the campaign, and to make them skittish enough to form poor war-mounts.
But if that fails, you could always add:
Pixie Dust. 50gp. When sprinkled over a creature, they may make a DC 10 Wisdom save to negate. Failing that, they:
01-20: Become euphoric for 1 minute; each round roll a d6: 1-4 take no action and move in a random direction; 5-6 repeat the wisdom save to end the effect. Creatures immune to charm are immune to this effect.
21-40: Become frightened of the closest creature for 1 minute. At the end of each round repeat the wisdom save. Creatures immune to fear are immune to this effect.
41-60: Cast invisibility on themselves.
61-00: Cast fly on themselves.