It has been a long time since I have been here so in case what I will be saying as already been spoken about, you can ignore it.
So I am currently replaying Eternum and everyone can tell that there is many greek mythology inspirations in it at least for the name. While I have finished to reread chapter 2, I want to speak about 2 or 3 things. These things are the vision that Orion as in his first night in Kredon which is the same one when he liberates Calypso, another is the first connection to Eternum.
Let's start with the first connection to Eternum. Orion fly over a sea and perhaps this sea is Oceanus. It could fit with all the others elements from greek mythology and all the souls that are trapped in Eternum. Oceanus is said to border Tartarus or Elysium, (the afterlife in general) and you have to cross it to enter these realms.
So, about the first vision / dream. I don't want to speak about what the vision show but why it happen. This vision happen the first night Orion is back in Kredon and the same one happen when he liberate Calypso. The reason why the vision happen could be because of the phone call in the booth, where a woman tell him to find Calypso and open the gates of Acheon. At first, we could assume that the reason of the vision is the mention of Calypso but I'm not quite sure. For one, the name Calypso is much more know than the name Acheron. Orion as surely already heard Calypso be it in reading greek mythology or movies using greek mythology (Pirates of Caribbean as an exemple). Since Orion is quite connected to Eternum and to the dead be it with how is body transform during the story, the thing that would connect more with him is Acheron and it would be Acheron which start the vision. He will have the same vision when he free Calypso because he need her to open the Doors of Acheron.
Acheron is a river of the underworld, which in some myths take the place of the Styx with Charon. In Eternum, or at least in the first 2 chapters, there is 2 differents mentions of Acheron. The first is the Doors and the second is the temple. I can't say if in the end if it's the same thing that the phone voice (door) and the Founder (Temple) speak of. Acheron can be considered a gate of the underworld which could feat with the Door. Perhaps the gates will be used the others way. Instead of crossing Acheron to enter the underworld (whatever is Eternum), they will cross it to come out from the underworld and let the souls free instead of trapped.
Well I'm currently done with this though, thank you for reading this.
Could also be that Orion is flying through a vast river instead of an ocean, and that said river is basically a river of the dead.
Also, from the looks of it, the story conveniently never brings up any direct connections between certain characters and their Greek counterparts other than during the costume party where Orion and Eros were mentioned (though there are plenty of references of the wink wink nudge nudge variety such as The Huntsman and Orion's obsession with crossbows being nods to his Greek myth namesake). There's also Psyche, but she seems to be referenced as her own thing at the moment rather than being connected to her Greek myth counterpart.
We do not know for certain if Orion's dream that we see at the beginning of the game is him dreaming such for the first time or something that he has been dreaming even before Kredon. Remember, he keeps mentioning that he doesn't dream even if he does, so who's to say he even remembers the ones he had before the phone call?
(Also there's speculation on Cari's Discord that the phone call might be a dream too or a mini-bleeding incident, since he conveniently hears the phone call while he was alone at the park)
My guess is Acheron could be both a temple and a gate. Or the gate/door is located within the temple. Or the temple is before the doors themselves and you have to perform a ritual in order to open them or keep them closed. Regardless, the name is likely being used liberally instead of being a direct reference to its mythological counterpart the same way that Ogygia is a snowy server instead of an island or Styx is a server mentioned by the Founder in Ion instead of a river.
Also, I'm guessing that door likely leads to Elysium, considering that Idriel keeps hinting that the dead aren't supposed to be trapped in Eternum despite certain people believing it to be THE afterlife (i.e. Luna's Great Aunt and El Coyote). Given hints that Eternum is the Astral Plane (or Cari's take on it), then it's understable why the dead are there to begin with and why it's connected to millions of different realities (it's mentioned in some texts that the dead pass through the astral plane in order to reach the afterlife). Why they are trapped there though is likely something we'll find out eventually in a future update (though it's probably because Acheron is closed and so the dead can't pass on to Elysium/the true afterlife).
