River of Woe, the mighty Acheron is one of five rivers, which flow into the Olympian Underworld. At its apex within the Greece of Midgard, it is a meandering green and fertile river. The further one goes along it, the harsher the environs. The less grows, until the banks of the Acheron are dried mud and stone, the only life scavengers which drift off soon after for lack of food.
An unforgiving and listless place, Acheron's banks carry the sound of moaning dead, or the ragged conversations of those, who have not yet decided they belong to the afterlife.
Or those as Icarus Areides & Leander discovered, who could not pay the ferryman's wage.
Fauna & Flora
While some historians bicker endlessly about whether Charon pilots his craft across the river Acheron or the river Styx, they can all shut their mouths and make a cup of tea. The River Acheron feeds into the River Styx.
What little history remains is this: Once someone of the Hellene faith is dead, Hermes acts as their Psychopomp to bring them to the banks of the river, where they are deposited with the knowledge they're dead and they'd better damn well have their coins or they're going to be very bored on the wrong side of the river without even the hope of reincarnation sometime when their shade wins the Pomegranate Lottery.