Eternal Flame (Greek)

Background
The Eternal Flame is  the same first flame Promethus stole from the gods and gave to humans. Keeping it in the Underworld was the only place it could not be destroyed. It represents all humanity has done of hundreds of years, and the spirit that keeps them alive.

Mentions
Leah Winters, a child of Hades, and her friends were once sent on a quest to take the flame, which was stolen by  the Giants, Minmas,( Mimas had serpents for legs and was born fully armoured.  He was slain  during the (first)  war against the Olympians  by a volley of molten iron) and  Clytius( Immolated by   Hecate  with flaming torches, also born with armour and serpant legs. They both wanted revenge for their death by flame and decided to take take the most important fire there was and use the fire's energy to bring down the gods.

The quest was succesful and Leah was given her reward, an slip of paper with Brandon Sonni 's address on it.

Hades also mentioned it while talkinng to Tula Bonham about her quest to save a huntress from Minmas.

What It looks like
No one know exactly what it looks like, as you can't stare into it directly, but a view rough drawings from the demigods on the quest show what it might look like. It is sometimes protrayed as a young girl.