Within The Conjuring Universe, The Nun will be the very first film in the timeline, taking place in 1952, although Annabelle: Creation does have some flashback scenes that take place before it.
The Nun represents the start of what the Warrens will battle decades later in The Conjuring 2.
The character of the nun was first introduced in The Conjuring 2, which took place in 1977.
The Nun will take us back in time to reveal the origins of that evil.