I'm not sure that I made the ending as clear as it should have been, so here's a quick explanation of the plot (minor spoilers for GoG lore):
So, this article takes place in the Gods of Griots storyline, which is basically about the titular Gods who once lived in the Backrooms thousands of years ago.
Basically, when a member of the Lost killed one of the Gods, and this event made everyone in their village immortal (see: Gods of Griots), this includes The Father, and another important character who will be explored later in the canon (note: in some head-canons, aging doesn't happen at all in the Backrooms. That is explicitly not the case for the GoG storyline).
David (the U.E.C. member in this article) doesn't really like the idea that he will eventually die, so when he finds out about rumors that some members of The Lost are immortal, he decides his goal is to find out how he can become immortal. Now, David doesn't actually care about about anyone else aside from himself, but he knows that if he says "I want to find immortality for myself", the rest of the U.E.C. are not going to help him, so he frames it as him just doing things for the good of the U.E.C.
Now back to our boy Entity 987. He's not exactly a God of Griots, but here's something very related, and he's been around for a really long time. His origin story is that he was basically a God who ruled over a small Lost group, and eventually they became afraid of his power, so they decided to trap him using some magic (basically they did a little human sacrifice, and the souls of the 11 people they sacrificed bind Entity 987 to the level), and now Entity 987 is super salty about it and wants to do a little trolling to the entire human race as revenge.
As it turns out, there was actually a little bit more to the translated document that David's team found that he didn't mention to Oleana. You see, the document also specified that the spell could be broken by burying 11 iron arrowheads in the ashes. Luckily for him, he happened to also find a box of 12 iron arrowheads, and while these particular arrowheads are a bit overkill for what he needed, they certainly get the job done (quick lore tidbit: these particular arrowheads are from a batch of 13 designed specifically to kill Gods of Griots. Only one of them was used, in the killing of Kayan [see: Gods of Griots]).
When David gets down to the chamber, he gets immortality from 987, and tells the guys upstairs to bury the arrowheads, but what he didn't expect was for Entity 987 to do a little trolling. Entity 987 then swaps their bodies (side note: the reason "David"'s hand is burned at the end where he was holding the arrowhead is because it's now 987 in that body, and 987 is burned by iron). Then, just after Entity 987 in David's body leaves, that part of the level falls away into the void (the reason for this is that, as an unintended side effect of the sealing ritual, that part of the level had a very weak connection to the rest of the level, and that connection was severed completely when the souls were unsealed).
So where does that leave us? Well, David got his immortality, in the end. Of course, it won't do him that well now that he's trapped in a cage falling eternally through the void, so maybe he should have heeded The Father's advice. And 987 is now on the loose, so I guess you'll just have to wait and see what he does next.
(Also there were certainly some details in here that I didn't go over because they won't come into play until later articles, and I don't want to spoil things, so look out for those ig)