You can see Ephemer talking to him just before that and it's parentheses. So I think it's an act.
Didn't even notice the parentheses among all the japanese symbols, lol. Good eye.
From what I gathered, Player pretended to join the darknesses so that Ephemer would forcefully send them and the player away. It was imperative that Ephermer and Skuld truly felt like they had to send Player away. Player teleported to a place that I guess made it easy to seal up themselves and the darknesses inside so they'd all die as the world ended. Player laughed at the darkneses because their plan succeeded
That sounds like something more like Player would do when recalling the original X[chi].
Didn't they state in the last update though that "the world ending" results in sleep instead of actual perma-death?
I guess though one should refrain from making too many assumptions just from visuals.
the player got possessed by darkness.
That's a possibility too, mainly because one Darkness is missing.
According to earlier statements there are
seven Darknesses and here we see only four.
One is sealed within Ven right now if I recall correctly while another went to the outside world with Maleficent and later spoke/fought with Luxu which means five of them must remain.
If one is within Player it would explain why only four are around visibly.