Upon first arriving in Dereth, I performed an errand for the tavernkeeper in Shoushi and he rewarded me with a key which would open a locked chest in the Underground Forest near Glenden Wood. I kept the key but never found the underground forest.
Fortunately, today Vanidar told me he found it. I fetched my key and joined him in exploring.
We found stone-hewn tunnels connecting lovely subterranean gardens populated by, of all things, banderlings and wisps. In one chamber, we found a chest, but it wasn't locked. So, apparently, we didn't need my key.
Maybe I can sell it for a few pyreals?