Well, that suddenly took a leap to WTF. As for Lovecraft: the comments mention Shub-Niggurath, who is also known as The Black Goat of the Woods with a Thousand Young. The giant creature at the end was probably meant to be Shub-Niggurath.
Selected quotes from the Lovecraft wiki:
Shub-Niggurath is an Outer God in the pantheon. She is a perverse fertility deity said to appear as an enormous cloudy mass which extrudes black tentacles, slime-dripping mouths, and short, writhing goat legs. Small creatures are spat forth, which are either reconsumed into the miasmatic form or escape to some monstrous life elsewhere.
The Dark Young of Shub-Niggurath are horrifying, pitch-black monstrosities, seemingly made of ropy tentacles. They stand as tall as a tree (perhaps between twelve and twenty feet tall) on a pair of stumpy, hooved legs. A mass of tentacles protudes from their trunks where a head would normally be, and puckered maws, dripping green goo, cover their flanks. [...]Dark young act as proxies for Shub-Niggurath in the accepting of sacrifices and the worship of cultists, in the devouring of non-cultists, and in the spreading of their mother's faith across the world.
What exactly happened in the video I do not know. It looks like the black-haired girl birthed a Dark Young, which then transformed that couple. Why she birthed that, I have absolutely no idea.
edit: alternative theory. That pool was a temple of Shub-Niggurath, and the sexual acts of the couple awoke her. She then tried to transform everyone in the pool into Dark Young, but it didn't work with the girl because she was a virgin. The second time the girl jumps into the pool, she goes through the portal to the plane of existence of Shub-Niggurath. Don't think there's supposed to be any moral or metaphor in that.
edit2: theory found on Reddit:
the story is created by the reluctant girl and after seeing her friends kissing she see's them in a different light, corrupted so to say and she see's herself as partially corrupted (maybe she knows what the night will lead to, or perhaps just by association she thinks she's corrupted, it does't matter).
So when she gets out of the pool and see's her corrupted friends she escapes leaving beer boy on his tod, because in her mind boozer bob wants what wormarm boy has got.
She tries to escape the impending doom and in the end after trying all possible avenues she dives into her "safe place" where the dawn breaks and the doom is realised, she's lost her innocence.
So to recap, loss on innocence or corruption.