Guild Wars 2: Heart of Thorns, the game's first full expansion, is one massive spoiler. So if you've been playing the game and aren't up to speed on the finale of Season two of the game's Living World update, you may want to give this story a pass.

In other words: Spoilers ahead.

Heart of Thrones takes place the day after the final episode of the massively multiplayer online game's season two. The season ended with a giant fleet of airships flying into the Maguuma Jungle to confront the Elder Dragon Mordremoth only to be ripped apart in a matter of minutes. Perhaps more troubling, though, is that many of the game's Sylvari botanical race are revealed to be under the control of Mordremoth and complicit in the destruction of the Pact fleet.

"This expansion begins within a day of that happening," said Colin Johanson, Guild Wars 2's game director. "You have to go into the jungle to dig in the wreckage and find heroes and then fight your way through the jungle to find and kill Mordremoth."

A narrow slice of the story was playable as a demo at PAX East last week.

The gameplay session opened with my hastily created character, one that is in the new revenant profession, cresting a hill to see the remains of the fleet being torn to bits almost immediately the characters on screen begin to talk. No cut scenes, just a conversation.

"We have characters talking in the world now," Johanson said. "In this expansion we have tried to make the experience more immersive. Conversations play out and it feels more natural. And characters have a lot more dialog because of that."

One of the big questions raised almost immediately, in this post-Pact fleet world, is whether you can trust any of the Sylvari, Johanson said.

"If you play as a Sylvari, you're constantly questioned about your motives, even by your oldest friends in the game," he said. "Whether you decide to trust a Sylvari early on in the expansion impacts what mission you end up doing."

In the demo, your only option is to trust.

Another twist is that with the fall of the fleet you end up becoming, at least initially, the highest ranking surviving member of the Pact so most troops you come across will follow your orders and refer to you as the head of the fleet.

Once the game progresses into the mission, the play isn't initially that different. But that changes dramatically in battle if you are playing a revenant. The revenant have the ability to channel a legend, which opens a whole second set of special attacks and abilities and you can swap which legend you're channeling on the fly. One of the neat perks that comes with doing this is that, since all legends have a healing ability, you can heal yourself twice in a row.

On top of the tweak to combat for the new revenant profession, comes a broader change to the game through the introduction of the master point system for characters who have topped out.

"Instead of adding levels or new tiers of gear we decided to add masteries," Johanson said. "We're never going to do either of those, this system is our replacement for that."

The team decided to avoid adding levels or new gear because they feel that by doing so it essentially invalidates all of the old hardware you already own.

"The mastery system allows you to add points for doing anything challenging like defeating really hard bosses, completing major story moments," he said. "Then you can spend those points for purchase to change the way you explore the map."

For instance, the first mastery I unlocked was the ability to hang glide, jumping off high points and then drifting down to a lower level.

Other abilities include bouncing, higher jumps and faster runs.

There are also abilities that teach you forgotten languages, allowing you to solve mysteries that will unlock areas of the map. Finally, there are some combat abilities which can be used against special enemies. One ability allows you to rip the thick armor-like bark from an enemy so you can damage it. Another allows you to remove the goop a certain creature uses to incapacitate you.

"The progression system is account wide," Johanson said. "They exist on all level 80 characters."

The publisher is still figuring out the pricing and packaging options for Heart of Thrones, and Johanson said they were too focused on seeing how this expansion goes to be able to consider whether expansions will become a regular thing for Guild Wars 2.

The expansion will also come with a new multiplayer game type called stronghold, which pits two armies battle to kill a guild lord, said Isaiah Cartwright, a lead designer on the game.

The game type forces players to tactically take on different roles if they want to win the game

"You have to get supplies, you have to summon NPCs, making sure they stay alive is a role," Cartwright said. "It brings out really different strategies in the game."