Crater and Curtain-raiser

Crater noun - A bowl-shaped geological formation at the top of a volcano.

Curtain-raiser and crater are semantically related. In some cases you can use "Curtain-raiser" instead a noun "Crater".

Curtain-raiser noun - A performance, activity, or event that precedes and sets the stage for the main event.

Crater and curtain-raiser are semantically related. Sometimes you can use "Crater" instead a noun phrase "Curtain-raiser".