'This information was obtained on the test server and is subject to change at any point'
The Hammerknell Fortress opening event has three special invasions that take place in Moonshade Highlands. During these events you will be rewarded with Rune King's Seal which is also the currency of the event. It should be noted however that other means of obtaining this currency and the vendors for it were not implemented yet.
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After 100 total shots have been fired at a given Cyst its barrier will drop. This will cause a Dread Stalker to spawn. The Stalker is level 39 and has near one million health, much like a typical Moonshade Highlands zone invasion boss. After dispatching all three Cysts and Stalkers the event will complete and you will be rewarded with 25 Rune King's Seal.

However, this event can spawn considerably more difficult rifts than a standard zone invasion. Expect to see minor rifts that have non elite mobs with 30,000+ health and bosses with 140,000+ health. The major rifts bosses can have near one million health.
When all of the required invasions and rifts have been dealt with this zone wide message is heard:
The invasion objective will update with Defeat the Abyssal Invader. Two bosses will spawn in the Highlands. One is the Bloodtide Fanatic who has around one million health and does nothing too special. The other spawn has a million health as well but after he dies Slaughter Jaw will spawn. Slaughter Jaw has nearly one and a half million health and does a lot of AoE damage.
After slaying Slaughter Jaw, thank your healers, and collect your 25 Rune King's Seal.

The Ascended must defend Reclaimer's Hold while dealing with these massive footholds. Each foothold has over 2.2 million health and a continuous stream of invasions to help defend it. After all four footholds have been eradicated Warden of Corruption and Tidal Destroyer will spawn and head towards Reclaimer's Hold. Here they intend to destroy each other, but not before killing every dwarf and Ascended that happens to cross their paths.
With the Warden and Destroyer defeated all that is left is to collect your 25 Rune King's Seal and head towards the gates of Hammerknell!