Your Garrison is your stronghold in Draenor, and powerful enemies have officially taken notice. As you level your Garrison, your followers, your buildings, and yourself, the rest of Draenor is watching and waiting for a moment of opportunity. Invasions are exactly what you’d assume—enemy forces attacking your Garrison in an attempt to tear it down. An Invasion plays out sort of like a scenario, in that you have several phases to complete, usually with a fairly tough boss encounter if you successfully complete all the phases.
You won’t be alone while you’re fighting off enemies. Your followers and Garrison guards will leap to defend your Garrison with you. After all, the Garrison is their home too! Successfully completing an Invasion offers special rewards. Failing to fend off an Invasion may end up in damaged buildings or injured followers and guards. Before accepting an Invasion event, you’ll want to make sure all your followers are present and accounted for, and leveled up appropriately.
Invasions also include bonus objectives, usually involving saving NPCs in your Garrison by escorting them to a point of safety. Completing an Invasion results in a rating being assigned based on your level of success. The three ratings are Bronze, Silver, and Gold. You can earn special Achievements depending on how well you do.