As this is but a theory at present, seeing as there is no way to either prove or disprove it in a reasonable amount of time, the contents are to be treated as such. It makes several assumptions about the universe (or, Multiverse, as it may be,) for it to work. Nevertheless, exercise caution when offered immortality, and get ALL the details, including any weaknesses, before even considering the arrangement.

You have been warned. Here goes:

The theory stems from the ideas of: the cyclical nature of time and the universe, biological immortality, the personal perception of time's passage, and maybe one or two other things.

Basically, the thought is that once one has achieved/been "granted" biological immortality (where the body no longer ages from the state it was in at that point, though it does heal, and female physiology theoretically will still work,) one will live on to see the end of time, presumably having also seen their own descendants prosper or die out.

Seems simple, at first, right? But it's not.

See, something I've noticed is this: as time goes on, our perception of how fast it passes undergoes changes. Namely, years seem to go by faster. There's a whole real-world scientific explanation for this phenomenon, but I won't go into it just yet.
So, say, the entirety of the 592nd century will seem to go by faster than the end of the 21st.

But it doesn't end there. You live to see how Earth ends, humanity's fate, all that. Perhaps humans, as a species, evolved past the need for physical forms, and became energy beings. Perhaps you have said effect artificially induced in yourself. You maybe even see the end of the universe.

For this theory's sake, we'll assume it ends in a big crunch, like an inverse of the big bang, and you retain all your memories as time goes on.

You see it end, and you survive it. And then, you see the universe reemerge, brand new once again. And you see the entirety of what transpires through time. And, again, the end comes. And the universe is reborn once again. This cycle repeats itself several times, and you begin to notice slight differences between each "timescape." You begin noting these differences, and labelling them.

Eventually, you notice that these slightly (or vastly,) different timescapes seem to have a sort of pattern to them, but you can't make it out at first.

Finally, you begin to notice groupings, how one type always follows or precedes another. Like the dots and dashes of Morse code, except far more advanced than such a pitiful binary means of communication.

You figure out "letters," and quantify those. They give way to "words," and then "sentences," and so on. You seem to have tapped into a sort of story, though you are as of yet unfamiliar with the language it is written in. It seems, in your mind, to sort of spiral around itself, like a helix, or a spring.

You begin to notice other, grander cycles, too, and slowly, slowly, you notice that you've long ago begun to lose yourself. Oh, you still remember everything, but... You're not exactly YOU anymore, not completely.

The "story" begins to wrap itself around you now, through you, as piece by piece, parts of your self are lost irrevocably. You eventually go insane, and just as you go under, just as you lose the last, final shard of your sense of "this is me," you see the final pattern, know what is happening to you, and what is about to happen, you understand EVERYTHING, you REMEMBER everything, past and future, and you're pretty sure now this isn't the first time this happened.

It's too much.

It slingshots your condition into overdrive, to a vegitative, brain-dead point.

Unbeknownst to you, for there is now no "you" left, the story, the strange letters, they begin to wrap themselves even more intricately through you. You are a blank slate, and they will use you to their own desires.

You are reborn, as a new person, on one of any possible number of worlds.

Fresh for another run.

Feel free to suggest anything you think fits here, people.

Dude. That's deep.

5126450 so to sum up, given long enough (IE tens of universal life-times), an immortal will become all knowing, but this will destroy the mind of the immortal, thus leaving them a blank slate.

their mind will then essentially "re-boot" leaving them a blank slate, a clean canvas, no long-term memories, no idea of who or what they are, only that they are. Just imprinted memories such linked to skills, such as vocabulary, ability to walk, combat-based skills, etc.

that is one hell of a long-winded and drawn out way to "wipe-clean" a character and start over, but if done right it would be a phenomenal story. both leading up to "the event", as the character slowly starts to lose sight of themselves and what it means to be that species. Then finally the journey to discover themselves again.

Sort of. It's more like... Complete reboot, both mental and physical

You're reborn as an infant. On one of any number of worlds, and as one of any number of species.

There's no possibility of recovering the "original" consciousness.

If you're familiar with the whole "last airbender" thing, and the avatar cycle, then you know how each new avatar has a unique personality, unique mindset. They were merely an embodyment of the elements they wielded, whose minds were just as u ique as those of the rest of us "normal" folk.

and you're pretty sure now this isn't the first time this happened.

So, theoretically, this plays in with another theory I have, about reincarnation: that all of us, every single living being, are merely different "points" on the reincarnation "timeline" of one single soul, or equivalent.