Prologue, Page 7.

Why yes, that is a Plague Doctor in Panel 3. Earth has always been shunned. Most attempts of travelling to Earth have ended in sorrow, and the last expedition fixed the taboo for a thousand years.

Explorers from Oborull traveled to a town dying of the Great Plague. They fled back to Oborull, but the damage was done. Some of the infection carrying fleas came back nestled in the fur of their faithful dogs. The devastation upon Oborull’s population was unprecedented.

The sign for Earth is the sun and the moon, with a skull to symbolize death.