From what we now until now, it's that the way passengers work, is by doing the spell, dying and then possesing the intended victim. A
From what we now until now, it's that the way passengers work, is by doing the spell, dying and then possesing the intended victim. A
(...) And then burning the "older" body, in order to gain full control of it.
So in a way a passenger can become inmortal by repeating this process many times.
So my theory is this: Celeste knew what was gonna happen, (getting herlself killed by Klaus) or maybe she just had a back up plan just in case, and she's been doing this all the time, meaning she's never been on the other side. That's why she was so sure (When being inside Sabine) she would be able to come back in first place, she had already performed the spell in someone, and knew that when dying, she would end up possesing that body. But once Elijah and Monique returned her to her own body, the spell or link was broken, so she could finally die, without being able to come back.