The sentence would be: "The guests had arrived before the rain began to fall, and they were all now assembled in the chief or living room of the dwelling."

The arranged sentence describes a situation where guests have arrived at a dwelling and assembled in the main or living room before the rain began to fall. Here's a breakdown:

1. "The guests had arrived": This sets the scene by indicating that guests have reached their destination.

2. "before the rain began to fall": This phrase specifies a timing in the sequence of events, indicating that the arrival of the guests preceded the onset of rainfall.

3. "and they were all now assembled": This connects the previous phrase and emphasizes that all the guests are now gathered or grouped together.

4. "in the chief or living": This specifies the location where the gathering is taking place, indicating that it's the main or living room of the dwelling.

5. "room of the dwelling": This completes the description of the location, clarifying that the assembly is taking place indoors, likely in the primary communal area of the house.

Overall, the sentence paints a picture of a social gathering where guests have arrived at a house, gathered in the main living area, and are now assembled together before the rain begins to fall.