Raid
A Raid is a special event that's frequency can be adjusted using Raid Frequency Settings
Before the raid starts, two "Raiders are approaching (settlement) from (direction)" message will appear as an early warning, then "Raiders are preparing at (settlement)" as the Raiders spawn in. The raid starts when all Raiders have spawned in or if they are attacked or aggro'd.
When the raid starts, the Raiders will march towards the settlement flag in waves. No more Raiders will spawn in.
A raid can potentially kill a minimum of 2 settlers, or up to many many more depending on your settlement size. Once the number is reached, the message "The Raiders are leaving" will appear and all raiders walk back to the edge of the map. See Raiders for figures.
Raids will end in two ways, either they hit the limit for killed settlers, or if all of the Raiders are killed.
If a settler dies during a raid, they do not respawn, but the same type has a very high chance to arrive soon after to be recruited.