Brain of Cthulhu
The Dryad is an interesting NPC indeed. Known to be the last of her kind, the Dryad is obsessed with the balance of the world. We know little about the other Dryads, for the rest of the Dryads perished in the Great War of Cthulhu after having weakened Cthulhu into the Moon Lord. So how did this Dryad somehow survive while the rest of her kind died out? Well, let's discuss!
What We Know:
The Dryads were an ancient race in Terraria with a strong connection to nature. The Dryads are the race credited with weakening Cthulhu. Unfortunately, all but one of the Dryads lost their lives in that battle. This Dryad moves in to a player-built house after the player defeats either Skeletron, Eater of Worlds, Eye of Cthulhu, or Brain of Cthulhu. From conversation with this Dryad, we know that she is 500 years old. Besides selling you things related to nature and telling you what percentage of your world is Corruption/Crimson, the Dryad also blesses the player when monsters are nearby. This heals the player while the player is within the leaf circle and causes enemies that attack the player to take thorns damage, as well as damaging monsters within the leaf circle. The health regeneration/damage amount depends on the number of bosses defeated in the world. This affect make the Dryad quite useful. Which brings me to my theory: Could these powers be the reason the Dryad survived?
Evidence and Reasoning:
I'm not sure exactly how the other Dryads behaved and whether or not the other Dryads also had these powers, which may make this section a little more iffy, but I'll do my best.
A lot of the items the Dryad sells happen to do with keeping the world balanced. From different powders to different seeds. The three powders she sells (Purification Powder from the Purity, Vile Powder from the Corruption, and Vicious Powder from the Crimson) each spread their own biome. Is it possible the Dryad used this to her advantage to keep the world balanced until we came along?
The Dryad's powers are one of the most interesting parts about the Dryad, as it can heal friends and harm foes. It still remains a question whether the other Dryads had the exact same powers, but regardless, this is a possible reason on how the Dryad survived. Perhaps she used her powers to keep the evil that killed the other Dryads from getting to her. I have no idea what exactly killed off the Dryads, and assume that Cthulhu did so as the Dryads weakened him, but other than that, nothing. However I do believe these powers are most likely the reason the Dryad is still alive.
Again, this is just a theory, and may not be 100% accurate and is not confirmed. Also this theory in particular is a little lower quality than my last couple, mainly because of the lack of information both
in-game and in the lore, but I still hope you guys enjoyed this. Thank you for reading!