Truth be told, the Shadow Realm is as much the Nymph's as the sunlight is the humans or the mountains are the Dwarves' -- they appropriated as they encountered it at the core of the millenial oaks that are their immortal ancestors.
The Shadow Realm is more dangerous than you may think when you see a Druid open it during their moon-bare nightly ritual. It is as if the shadow of the oaken mother ripples, like a pond after a dryad kissing you returns to her watery home.
A druid can learn the ritual to call forward an opening to the shadow realms, but at a price. Usually it is a day and a year in service to the nymphs, an ordeal that ages humans by decades (1d10x5 years); elves can only learn it by offering one of their offspring into eternal bondage; very few are willing to pay the price. No-one ever heard of dwarves or halflings being privy to the secret ritual, which can be observed as follows:
The Druid opens himself to the moon-bare night, stripping bare of all cloth and being lit by the light of the stars alone. He brings forward a copper sickle, which he uses to cut seven leaves of poisonous oak. He chews one of the leaves (save vs. poison or lose half speed, initiative -2 from terrible cramps for 1d6 days), and all who want to join him have to chew one as well (save vs. sickness or lose half constitution until treated). He can then lead those who keep on chewing through the Shadow Realm on the path he knows. One can search for new paths, but this is a dangerous ordeal and many were lost._
Known pathways have a silver glow that is easy to follow, and learning the ritual includes knowing one path (like the one described in the hex). Other paths may require other preparatory rituals, and trying to find them requires a save vs. magic or roll 1d8:
- Lost in the Shadow Realm. Save vs. magic each day to find the original entrance. If you don't bring water and food, you may well die.
- Lost in the Shadow Realm. Appear at (a randomly determined) established target location after 1d10 years, while the journey took 1d10 days.
- Shadow hounds hunt you back onto your path (Number: 4, HD2, AC3, immune to nonmagical weapons, on successful hit save vs. poison or lose 1 CON in addition to damage).
- Lost souls of people that died in bondage to the nymphs beg you to free them. If you agree, they take your body, and your soul is stuck here in their stead.
- Lost souls of people that died in bondage warn you of the dangers of the Shadow Realm. They can be quite talkative, but getting meaningful information out of them may require a beneficial reaction roll.
- A lone shadow hound pup takes the liking of the characters on a beneficial reaction roll. Spellcasters can try to win them as familiars; thieves treasure them for being able to hide perfectly in the dark; fighters love their sickening bite when their foes suffer it.
- Be touched by the Shadow Realm. Your shadow is now animate and possibly self-willed. You end up at the origin of your travel.
- Be touched by the Shadow Realm. You can turn into shadow for 1 phase/day. You end up at (a randomly determined) target location after 1d10 days, while the journey took you one day.