Continual light/flame - how common are they?

So I was prepping this adventure in a castle where I figured that several rooms would be dark - the servants have been prevented by mayhem (why the PCs are called in) from lighting or replacing candles, torches etc. Then I realized - silly me, this castle is owned by a powerful wizard, there should be continual flame spells everywhere.

(In several editions of D&D, there is a "continual light" spell - a light that lasts forever.  There is usually a cost (in 5e, it's 50 gp's worth of ruby dust), so you can't do it willy nilly.)

But then I started thinking about it... Continual flame spells are expensive and should be rare... or *are they*?

Let us consider not a mage or a rich noble, who of course can afford it, but a modest artisan. He's doing ok for himself, living a lifestyle of 1.5 gp a day (halfway between modest and comfortable by 5e standards). He needs light every evening in a single room, for 3 hours on average. Nothing extravagant. This, however, has a cost. If he uses a lantern or lamp, this is about 5 cp/night. Candles would cost him 3 cp/night (and shed less light). Torches are as cheap and shed more light, but the smoke... so let's stick with an oil lamp - he's has a little bit of money, after all.

At 5 cp a night, this adds up to about 18.25 gp a year. So in other words, a continual flame spell would pay for itself in less than 3 years! People in the middle ages were capable of long term planning - they did long term projects for great gains - building a fence, planting an orchard, or building a cathedral. Our artisan could, for example, limit himself to candles and in less than a decade, take the spare 2cp/night to buy the continual light, and save that 5 cp a night for other things.  The spell is safer (no fire risk) and sheds no smoke. And his light can be passed on to his children etc.  

I do have to note that this is just for the material cost, not the fee to the caster.  But even if the cost is doubled, it's still a good deal.

Given that continual light spells can be cast by low-level casters, I can see this as a common, harmless way to raise funds. Want to fund your magical research but low on funds?  Make some magical lamps and tell them to the local baron!  Temples could sell them too to the faithful, those cathedrals need maintenance - they might be hesitant to give magic to the masses, but (same as a potion of healing), what harm could *light* do? Even very humble peasants may have one - the "family continual flame", passed down from generation to generation - it was given to great grandfather Jeb by the bishop as a reward for his help in fighting off the goblins - or some other colorful story. 

Having continual flames everywhere may be too "magical" for the setting you want to create. But the economics tell us that, unless casters are *incredibly* rare, they should be all over the place.  

Another quick bit of number crunching.  In the kingdom of Notsomagicland, there are 10 million people, but only, at any given time, 100 casters (on average) capable of casting continual flame.  They only deign cast the spell once a year, and the spell has a duration of 1000 years (it's permanent, but accidents/loss happen).  So... with this limited number of casters, who rarely use their magic, there are 100 000 magical lights around at any given time... Now imagine if there are 1000 casters.  And they cast it 1/month... 

Given that in Yoon Suin, the ruling class are almost *all* spellcasters and it is a very large city... you're going to have a lot more than 100 casters.  But I think that in Yoon-Suin, continual flame would be seen as *boring*.  Oh sure your room is lit by continual flame.  I use candles... made with yakmen tallow.  You can smell their anger!

Interestingly, in Veins of the earth, light is incredibly precious underground, and 1 gp = 1 hour of light.  So a continual flame would be priceless.  I imagine for that game the spell would have to be modified, otherwise the economy doesn't make sense.  And that's ok!  Change away, it's your game.  

Grown in Darkness

The other day as I was surfing the internet, I stumbled upon this story:  

http://www.bbc.com/travel/story/20190424-the-english-vegetable-picked-by-candlelight

In brief, someone discovered that if you forced rhubarb to grow by candlelight, it would force them to draw upon their resources and become much sweeter.  I'm not sure if I'm being a botany geek here, but this seemed remarkably like a... very Yoon-Suin thing to do.  Sure you grow bizarre vegetables in the dark in D&D but in the real world?  No way...  

And yet, here we are.

So in honor of forced rhubarb, I guess I have to force myself to write down a few ideas I had about the power of darkness in Yoon Suin.

First, yet another god!

Lyla, the dark soap mistress.  This peculiar cult is not very popular due to being... strange, even by the Yellow City standards.  The goddess takes the shape of an odd Beetle-octopus, and appreciates the sacrifice invertebrate.  The sacred color is black, and the cult takes the devotion to this color to an alarming extreme.  The priesthood is composed entirely of  human women who dye their skin black. The more advanced the holy woman, the deeper and more total the shade (teeth, eyes etc become black too).  Other cults consider this concern over color to be over the top. 

The high priestess has become so dark that she is but a silhouette, appearing not as a black shape but almost as a human-shaped hole in the universe (Look up vantablack to get an idea).  She is said to be one of the most powerful holy woman in the city, and slugmen sometimes pay great fees to consult her on various matters.  

High rituals of the cult includes sleeping in ink, sometimes for very long periods.  Other strange rituals include very long washing (most people aren't "holy enough" to withstand this, skin gets irritated, although certain slugmen say it is just *the thing* to get over an opium addiction).   Soap is seen as holy because it reveals (the sheen of soapy water gives substance and detail to very dark objects/people) , but also the soap is needed to keep dust etc at bay that would keep true blackness from being possible.  

Besides the high priestess, another cultist gained some notoriety - the would be successor, Chi Da Mat, was banished for her greed.  She relocated to Ras Bolon and quickly took over  the cult of the Black Lotus, a different goddess concerned with magic, women and the harvest.  She is said to wield not inconsiderable mystical and temporal power, and charges dearly for her services.

Second, a peculiar tea:

  

The unseen brew.

This black tea is grown in the shade, on steep northern slopes in Sughd that never seen the sun.  It is the harvest process however which is most particular - only on moonless night, cloudy if possible.  The harvesters are "honored" slaves, who are born and live in perpetual darkness, only coming out of their lightless halls to perform the harvest.  The same slaves perform the post-harvest processing, fermentation and packaging in complete darkness.  Each packet is carefully sealed against light contamination, and is sold for 10 rupees (2 gp) per packet (one packet being sufficient to brew one teapot).  

The tea is said to taste best if brewed and drunk in complete darkness, and is the preferred brew of cynics.  It is also used by serious-minded slugmen who wish to ponder "what could go wrong" with a project, as it blackens the mood and strips the drinker of unwarranted optimism.  It is also said to be a suitable base medium for various types of darkness magic.  

(edit:  this was in part inspired by the imperial silver tips which really should be part of yoon-suin anyway.  )