Stygian Coin, also known as Charon, is a unit of currency.
Stygian coins are the most common currency around South Underrail, and they're accepted everywhere in the south. A stygian coin has a base trading value of 30, making them three times as valuable as SGS Credits.
They were first introduced as the currency of Junkyard and mainly used there, but they're also accepted as currency in the South Gate Station and in other South Underrail areas.
Charons are used just like SGS credits and other currency items, in that you can exchange them for weapons, ammo, etc. Currency items are always valued at their base trading value, without any trader's price modifier. They have no weight in your inventory.
You can exchange SGS credits to Stygian coins at Colton in Junkyard docks. The exchange rate is 5 SGS credits to 1 coin. This rate can be lowered to 4 SGS credits to 1 coin with 40 active Mercantile skill.