That being said, i'll briefly cover a few things that didn't get expanded upon in the chapter 6 intro.
Purification. Sealing. Exorcism.
These are the means of dealing with the enemy addressed, IE: "The Darkness". If we are counting the hybrids in any capacity, then Eth's All's End: Paragon of Light "purified" the threat neutralizing those masses of "darkness" completely.
Sealing. This is actually canon, and referenced in the source material as the Master of Master's original plan to seal the darkness within himself and his six apprentices, stagnating the number of remaining enemies to thirteen. It is assumed this is outcome came to be, and we are essentially "Vessels" for the remaining thirteen based on the information Shiki provided from her investigation. Theres been no means (in canon anyway) that explicitly explains how this is possible to achieve, but without spoiling anything, we already have the means to perform this option.
Exorcism. We know this to be the power to remove something from the world line in addition to negating the effects of Omni or as we know it, Resolve.
The All's End technique is cameo as far back as Deimos first encounter with the Lingering Will at the beginning of the roleplay. It served to be the original teacher of the technique in this roleplay from the start. Given only two people (Saebyeolbe and Eth) have had a confrontation with it, they were technically the only two that learned it before everyone else did. Per the time-skip, everyone on the roster has learned it.
So what is the All's End? It is this roleplay's version of a limit breaking technique inspired by Birth By Sleep's Lingering Will vs Terranort encounter. The user creates a space that they have complete and total sovereignty over that is of a light affinity as it is based in light. It's seen as a last resort to completely overwhelm an opponent caught within your individual Resolute's All's End.
Naturally (only) in a training environment, can a Resolute use their All's End against another resolute. Here is where it gets interesting. The two different spaces actually overlap and will eventually cancel each other out automatically after a duration of 4 minutes {240 seconds) of both being active at the same time. Something to be mindful of. At this time, you are alloted to create 4 different variations.