As you play on the Island, regardless if your logged in or not, a bar will increase, it will increase faster the larger your base is, and the more eldritch horrors and vault dungeon monsters you’ve slain recently.
When it hits its maximum, it will spawn a mini Maelstrom called a Rift over your base, spawning various weaker, but still strong, eldritch horrors and various vault monsters from nearby vaults to assault your base.
The Rifts difficulty will be based on the size of the base, which the current difficulty will show on your bar. Any base structure can be selected though, so like purges if you have many bases or bridges, you may end up having to defend them.
Majority of Enemies will spawn outside your base, but there can be Deviants that spawn on top of your roofs or ceilings.
But, the reward is, sometimes crafting thralls rank 2-3 can spawn in the Rift as well, and they are hostile to the Eldritch creatures. Will you be able to save them and knock them out before they are torn apart?