Uhh, correction. Calypso can travel to any world she's been to before, not to worlds that she hasn't been to. The exceptions to that is emergency portal jumping (which she has no control of as far as the destination is concerned) and opening portals leading to people whose presence she has tracked/is familiar with (which made her crucial in the Heist, as it got Alex, Annie, and Orion (MC) out of their predicaments without issue). It's likely that the latter has limitations (that being that she could track people she knows WITHIN Eternum, not outside its boundaries (more on that later)) given that she's still not at full strength though. Later on in the story though? Well, weirder things have happened.
Also, it's highly likely that exit portals only work on player avatars, not natives of Eternum like Calypso. It's possibly designed that way to prevent anything from tracking down the realm/dimension where the players are coming from (thereby preventing what I would like to call an "Astaroth" event).
Or Eternum IS another name for the Astral Plane, player avatars are actually magitech-made astral projections/souls, and excommunication basically cuts off one's consciousness/soul from the body, leading to brain death and having said consciousness/soul trapped within Eternum and becoming brainwashed into an NPC. The Syndicate is able to track Orion down because of his perfect compatibility rating, and start trying to communicate with him through various means (mostly dreams, because as the Space Station audio logs have implied that seems to be one method of travel between Eternum and our world...which is also similar to the information mentioned regarding travel between the Astral Plane and the Physical/Material Plane).
Jury's still out as to what Orion's backstory really is. I originally was on the camp that believed (well, I still am I suppose but I'm willing to be proven wrong) that he and Thanatos were Ulysses lab rats who got experimented on as kids, with the findings of said experiment leading to the creation of the neural implants. With the hints from 0.4 though, another possibility seems likely (and is theorized by some in this thread): that somehow somewhere, Orion died but was brought back to life. Originally I dismissed this because I thought it meant that he somehow got himself killed and was brought back (maybe through magical/necromantic means) and Annie and Chang and even Orion's dad never knew about it. Now though...I'm thinking that maybe it's less of that and more of "he got into an accident - possibly back when he was a kid - and went clinically dead for a moment and got so far as to travel through Eternum and to the pearly gates of Acheron...but before he could really cross over he got yoinked/kicked back into the living and walked it off before anyone could ID him."
This of course is another possible explanation to the compatibility rating. I theorized a while back that it's the rating that dictates how attuned you are to Eternum itself (either through exposure or bloodline-related, hence the thing with Alex's compatibility rating and Calypso originally mistaking her as a half-elf...maybe there is some truth there), possibly meaning that Orion was at some point exposed to Eternum, possibly through Ulysses experimentation with the neural implant. Now though, there's another possibility: his brief brush with death and stay in Eternum has marked him as someone that's basically an "honorary native," maybe even marking his soul as something similar to the members of the Syndicate/NPCs.
As to how said explanation ties with Orion meeting the Founder when the former was a kid...the most outlandish theory I could think of is that Orion met the Founder...in Eternum (0.4 seems to hint the possibility that the Founder doesn't exist in the Physical/Material Plane). That is, Orion basically bumped into him while he was traversing through Eternum on his way to the afterlife (or, while he was looking for a way back into the Physical/Material Plane). The Founder then either wished him luck and sent him on his way...or was the person that sent him back into his physical body, possibly marking him as a future ace-in-the-hole that he could use in his schemes later on.
I'm on the camp that believes that the Syndicate are former NPCs who somehow managed to get their neural implants fried somehow, which led to them becoming free but then find out that they couldn't cross over into the afterlife, and now they're trying their best to find anyone that could help them achieve said goal (maybe someone that they can see...like a person with a perfect compatibility rating, perhaps?).
Just when Orion goes full berserk he's probably extremely resistant or borderline unkillable. While Thanatos seems to be killable. Just very very very hard to. As he said himself, Orion made more damage then most, yet, just, not enough. So there might be 'enough damage' at a certain point.
