(Spoiler Warning)
In the True Ending, the ??? character says the words "before this play" while talking about you killing Morshu.
This theory is that the game you are playing is a play, however this is most likely untrue.
I believe they were saying "Before this playthrough." Saying that you killed Morshu before this Playthrough
I'm not sure if they only say this if you killed Morshu on your last playthrough, but idk.
Please reply with any things I missed!