This is the conclusion (for now) of the story that began with ⁠The Last Halloween, part 1 — you should start there first. To recap:

Tasha has entered into a virtual "experience" of the end of the world, which an android refers to as The Last Halloween. The apocalypse continues, with everything around Tasha being destroyed. The experience overlays her memories so completely that she isn't even aware that she's not actually living through the last Halloween, a holiday she had previously only heard referenced in holos and books.