The Garden of Shadows

From the introduction:

No one is certain for what purpose Level 12 was originally built. All traces of that form and function were obliterated centuries ago when Phaedor Rhendelmar laid claim to the level in order to test his theories about ways to allow surface plants to grow underground. Many considered the half-elf druid to be mad, and the single-minded devotion with which he pursued his goal certainly seemed to bear that out.