I'd like you to explain to me the use of the gerund in the following two sentences.I included other sentences I'm not sure of.

Question

1) One day, Prospero's frieds are having a party. They only stop when the pendulum is striking the hour.
2 The protagonist is thrown into a dark dungeon. He explores it and finds out that its shape is square and has a deep pit in the middle. The wall is covered with iron and with horrible figures.
3) Can you say "The protagonist is on the brink of a pit?"
4) At midnight a strange man with (or in??) a red costume and a death mask turns up and scares everybody.