In Greek mythology, they were the nymphs or spirits protecting oak trees. Eventually the term dryad came to mean any kind of tree nymph.
Eurydice is perhaps the most famous mythical dryad. She was beloved by Orpheus, a famous singer and musician. However, on the day they got married it just so happened that Eurydice accidentally stepped on a snake. The snake promptly bit her, and she died.
Orpheus then went on a quest down to Hades in an attempt to get her back. He put on quite a concert for the king and queen of the underworld, so they agreed to allow him to take back Eurydice, but under some conditions: he was to walk in front of her and never look back to see if she was following. Of course, like an idiot, he looked back and Eurydice was promptly sucked back into the underworld, while Orpheus was never allowed to go back there again or put on another concert there (maybe his concerts were attracting too many hipsters).
Eventually Orpheus encountered a savage group of Maenads (drunken women who were prone to randomly killing things and having wild orgies) and was killed by them.
In some interpretations of the myth this was the punishment decreed upon him by the gods for not simply killing himself after his true love had died but instead trying pathetically to get her back. Thus he was killed by a group of women — how embarrassing for an ancient Greek dude.
On a side note, more heroic heros than Orpheus did manage to have more successful dealings with the underworld.
Hercules once strolled into the underworld and dognapped the three-headed hound Cerberus, with permission from Hades himself.
Hercules wrestled the fearsome dog and captured it with his own brute strength and thus was allowed to borrow it. (He later returned the hound).
The New Dryads
I made some additions of my own to this mythology concerning dryads specifically for my novel.
The tribe of dryads described in my novel resides deep in the Amazon rainforest.
These dryads are wild, boisterous, and whimsical. They obey no one except for the Great Tree which has watched over their forest for millennia. Occasionally, they also obey the priest of the Great Tree.
Oh, did I mention that there are male dryads as well as female dryads?
This is not very true to the original mythology, but I would rather they reproduced sexually for it will lend much more excitement to our narrative.
Amazonian dryads can live up to several centuries, and they’re about five times as strong as humans. They are also experts in climbing, dancing, and magic. They have beautiful round green eyes and greenish skin. So on the whole, it’s pretty good to be a dryad. If only the nefarious Timber Corporations didn’t threaten their forest!..