The struggle with their curse may mean Darklings try playing with light and photonegative space as they try to control their curse. This may lead to them, in their passageways, placing down stain glass markers. To the average person it would be junk lying around, but shine a light on it and it could make a pointer or even display a map intricately carved into the glass.
Darkling Rituals
the ritual's mortality rate is unknown; this could be due to lack of resources for research amongst Darklings, or because the ritual's success rate is carried by the individual in question.
This might mean a Darkling needs high constitution (to be durable). This could alternatively mean Darklings can't have too little nor too much experience or light absorbed, but definitely a great deal of light/experience considering their culture only has some of their most experienced undergo the ritual.
Darklings having only some of their most experienced undergo the ritual could be emotional or practical in purpose.
1) If practical, it would be due to the ritual's success improving if enough experience is had.
2) If emotional, it would indicate an emotional maturity wherein the Darkling has lived long enough to feel content or satisfied with their life to where perishing wouldn't bother them anymore.
The origins of the ritual aren't known, though it is speculated to be some lost/forgotten history between Shadar-Kai & Darklings seeing as how they are so similar even to the innovation of tattoos with functional purpose. (Shadar-kai used tattoos originally to ward off Shamblers, guardians of the Shadowfell, before they were accepted by the realm therefore no longer needing the tattoos and turning them into a fashion instead.) The Drow might be most likely to have records or know the answer, since Drow are the only ones who voluntarily exiled themselves. (Shadar-kai were banished with memory loss, but the interaction might've occurred after the banishment and therefore Shadar-kai may be just as, if not more, likely than Darklings to have records since Shadar-kai had a more suitable environment to play with than Darklings for it.)
Society
Elders are very much the "elites" of Darkling society, but this status doesn't correlate with their economic class. One reason is because gold cannot be thrown at the ritual for it to succeed. Second is due to most Darklings appearing monsterous, they struggle with trade in the material plane much the same way orcs and goblins do with human an other similar settlements.