Spoilers: Full spoilers from Chapter 1

The only really expunged spoiler from the summary is some of the information about what appears on the roof to Luke and Kota. However, we are not given a clear answer or description of what it is. I have 2 guesses, but first I'm going to exactly quote what the game says. For context, Kota has just called out "If there's anyone there, then show yourself...!"

It can be seen, prowling like a starved beast just out of sight. It can be heard, thrumming in the air like the fading voice of a lyre's plucked string. It can be felt, smelled, tasted. Like the chill of ice and the warmth of fire sinking into one's bones in turn. Like the sour stench of rot masked by floral perfume - crocus and asphodel. Like the coppery sting of blood mixed with the burning sweetness of wine. The Labyrinth leaves its mark on everything.

I have two guesses: either this is similar to whatever smell the Master gets in the finale of Chapter 18 when P and 'Jean' are acting crazy (thus may be the influence of another God) and thus what landed on the roof is the agent of that God that also caused Chapter 18 (I have to make a different spoiler thread about birds, we've had a few come up in the story that didn't immediately attack Asterion (normal birds should) and I think this is thus a bird since it landed on the roof and all) OR it could possibly be the Narrator, Nemesis showing herself to those two. Mostly the asphodel smell makes me think it may not be as evil as my first theory, since they're so tied to Hades, but why we don't get to see more about this creature if it were Nemesis, I don't know.