Spoiler: I think this is the last alter that "woke up" for Darlene. This world that we just saw is all in Elliot's head, and it's where the 3rd has been keeping safe.
Elliot's bringing all his alters together, and he's going to "wake up" as a whole person soon. Where he physically is, and what's actually happening around him when he does though, I'm not sure of. He may have actually shut down Whiterose's machine, or he may be about to die in a nuclear explosion, I'm not really sure about that, but that part we saw at the end was most definitely his personalities coming back together.
At the beginning of the show, he's supposed to go to Angela's birthday party, but can't bring himself to do it. In the dream sequence episode, he's marrying Angela, and she says he's only a month old. I think he created this alter because he was struggling with seeing Angela with someone else, and so he put all his hopes for a happy life with Angela into this alter, put it to sleep, and then the show began. I wouldn't be surprised to learn that Elliot and Angela had dated for a while, and that part of this is him coping with the end of the relationship while trying to stay friends still.