The Nature of the Horizon of Existence

Can't really think of a way to open this, so I'll just say it straight out. I believe that the Horizon of Existence is a gathering of Nihility path energy.

For one, the times we've seen it in the story have been when a character was approaching The Nihility. With Firefly, we saw her questioning her resolve and whether she had the strength to defy her fate. With Aventurine, he questioned what the purpose of life is if everything will one day return to nothingness. With TB, it was the final test before we went to face Sunday. In every case, those in front of the Horizon were presented with the option to give up, striding close to The Nihility. And in each case, we saw them turn away, rejecting The Nihility and thus preventing themselves from self-annihilation.

The other thing is, according to the Data Bank entry for the Horizon, only Self-Annihilators (who have encountered The Nihility) can actually see it. Now, on the one hand, this is most likely untrue. Firefly and TB have visited the Horizon, and Aventurine has definitely seen it, but none of them would appear to be self-annihilating at the current moment. Even the data bank entry itself mentions this as being potentially fabricated. At the same time, there does seem to be a bit of truth to this. As mentioned previously, those who have seen it in the story saw it at times when they were dangerously close to The Nihility, and Acheron is obviously already an emanator. If we assume that it appears to people either close to or striding along The Nihility, it being path energy would explain why it doesn't appear to others. We've already seen that pathstriders are more sensitive to their path energy than the average person. In 3.2, when Fuli's gaze falls upon Amphoreus, Himeko seems unable to tell while Black Swan easily feels it. It's not hard to believe that the same would apply to other pathstriders, including those of The Nihility.

Overall, I think that with the information that we currently have, the Horizon of Existence being a surge of Nihility path energy is the explanation that makes the most sense and explains its nature the best.