Heliopolis was a city on the planet Alpha Eridani II.
In 2156, ten women were knifed to death here. In 2267, this was discovered to have been instigated by the Redjac entity. (TOS: "Wolf in the Fold")
This city was only mentioned in dialogue.
Heliopolis ("Sun" + "City") was a Greek word from antiquity, and was the name of a city in ancient Egypt