Garrison Invasion

A Garrison Invasion is an event that occurs inside your garrison from time to time. After an introductory invasion triggered by questing, further invasions become available after killing enough of a certain type of enemy across. During the event, large amounts of enemies will enter the garrison and attack guards and structures. The invasion has several phases spanning about 10-15 minutes total. Success is based on a Victory Point (VP) system based on killing invaders and completing other objectives. Reaching certain VP thresholds grants up to four tiers of rewards, each of which can be earned once per week.

Triggering an invasion
Killing enough members of one of the following groups on will trigger an invasion at the garrison:
 * Ogres
 * Iron Horde
 * Goren
 * Killing mobs and finishing the daily quest at Broken Precipice contributes to triggering a Goren invasion, despite being occupied by ogres and ogron.
 * Botani
 * (Horde only)
 * Botani
 * (Horde only)

Kills made inside instances do not contribute to triggering an invasion, nor do kills of the Iron Horde in Tanaan Jungle. Kills made while that type of invasion is already available do not carry over, but as soon as an invasion is completed, the kill count is reset and that invasion type can be triggered again. There is no known limit on the number of invasions that can be triggered and completed per day or per week.

A common way to trigger invasions is to complete Garrison Support daily quests which require defeating enemies and completing other objectives in specific areas of Draenor. It is possible but unconfirmed that completing these quests gives a bonus to the hidden "threshold" for triggering an invasion. Scouting Missive items can be purchased using Garrison Resources to gain access to such a quest when it's not currently offered by the War Planning Map.

Rewards
Depending on the score achieved during invasions, rewards range from bronze to platinum:
 * Invader's Abandoned Sack
 * Invader's Damaged Cache
 * Invader's Forgotten Treasure
 * Routed Invader's Crate of Spoils

After completing an invasion, reward bags of the tier achieved and all tiers below will be placed in the character's inventory, if the character has not already earned that bag for the week. For instance, a character who has previously achieved a Bronze invasion earlier in the week could earn the Silver and Gold rewards the same week by reaching Gold tier in a later invasion. The number of invasions that can be completed in an effort to earn all 4 rewards in a week is only limited by players' ability to trigger them.

Regardless of score, one of the following boss items may also be obtained as a reward for completing an invasion:
 * Heart of Oak (Teluur)
 * Runed Greatstone (Gaur)
 * Bloodied Iron Horde Banner (Commander Dro'gan)
 * Legion Beacon (Lady Fleshsear)
 * Void Prison (Annihilon)
 * Arcane Highmaul Relic (Mage Lord Gogg'nathog)

Achievements
Individual invasion types:
 * Shadowmoon Clan Invasion
 * Shadow Council Invasion
 * Ogre Invasion
 * Iron Horde Invasion
 * Goren Invasion
 * Botani Invasion

Achievements for a single invasion:
 * Save Those Buildings!
 * Invasions Are Better with Friends
 * Bronze Defender
 * Silver Defender
 * Golden Defender
 * Platinum Defender

Achievements earned across multiple invasions:
 * Master Defender
 * Impervious Defender
 * Invasion? What Invasion?