Greg isn't getting it, Sans thinks, but that's fair, because he hasn't even told him everything he knows yet. Which, he probably has to, now. If this conversation is going to continue, he can't just leave out the details.

"This half angel came to town, to talk to us about how everything's been all topsy turvy lately. She said that it's all about the Severing event from twenty-some years ago - that something big enough, numerous enough, passed through. Enough that it was more than the wards that caused the Severing could take, and now everything's all screwed up."

"And it sounds like those otherworldly things were actually people." He pauses in his rambling long enough to face Greg, with the kind of expression that seems like it should be accompanied by tears of frustration, if Sans was any better at emoting. "People like us. That we just... got inserted here like a week ago, and think we've been here forever when we actually haven't."

"Some guy replaced Marmaroth completely! He didn't remember anything about this place. He said he was a dimensional traveler that just showed up here, like a ninja samurai or somethin'."

Giving details about Marmaroth only confuses matters, but the fact that it is this bizarre seems important. It can't be understated how fucked up that is.