Also often heard with direct object “Requiescat in pacem”, although “in pace” (prepositional object) appears correct. The nominative case for peace is “pax”; in that form we have it in English in expressions like “Pax Christi” or “Pax Americana”.
“Requiescat” is a verb form, indicating desire for the action to occur (”May he/she rest”).
Introit for the Mass for the Dead begins “Requiem aeternam, dona eis, Domine” meaning “rest eternal grant them, Lord”. Here “requiem” is the accusative case of the noun “requies”, rest. From this we get the English borrow, “requiem” meaning a musical genre.
Thanks for the lesson in Latin. It’s been a long time!