Pthumeru was an ancient city built by superhumans (the Pthumerians) who uncovered the secrets of the god Truth.
Pthumeru is deep underground. It was dug out, intentionally built as a civilization underground. It is so for the purpose of escaping the Blood Moon, the thing that brings out humanity's beastly nature.
Pthumeru Ihyll was the title of both the Phumerian monarch and its capital.