Board Thread:General Discussion/@comment-1031724-20170228222249

Having watched Doctor Strange today (for the second time, since I saw it in theaters), I caught some extra details and I think I get how it works now. (Spoilers for anyone who hasn't seen it)

The "eternal life" thing of the Dark Dimension comes from it existing beyond time, meaning no one can age in there. This is why the Ancient One uses its power to keep from dying of old age.

When the Dark Dimension was starting to consume Earth, any buildings it consumed were disintegrated. Best I can gather, Dormammu destroys anything he consumes while combining their particles with the Dark Dimension, or possibly himself, and the process kills anything living it absorbs.

The Ancient One implies that one will still exist within the Dark Dimension because she referred to the "eternal life as part of the One" as torment. However, when Kaecilius and his Zealots are sucked in, they start to disintegrate, and last we see them, they're still slowly dissolving as they drift through the Dark Dimension. So, if the "torment" part is true, then she probably meant their souls would be sealed inside the Dark Dimension (or Dormammu) after their physical deaths, like that creature that takes the souls of whoever it kills (ie: Freddy Krueger).

There may yet be some evidence that will come up and reveal something we don't know, but from what was said in the film and what we saw, my interpretation is that when Kaecilius was absorbed into the Dark Dimension, he was destroyed, dissolved into particles which, possibly along with his soul, are enfused with the Dark Dimension, and Kaecilius's "eternal life" is really eternal damnation.

Though much like Red Skull, his fate is somewhat ambiguous 