In Aion, after the events that led to the destruction of the Tower of Eternity, the world of Atreia was torn apart and is now divided into Asmodae and Elysea. Between those two parts lies the Abyss, a shattered wasteland containing the remains of the Tower of Eternity, now home to the Balaur, Aion's hostile NPC faction.
The Abyss is one of the game's core features and represents the main Realm-vs-Realm zone. It is accessible for players of at least level 25 and requires a quest chain to be completed before you can enter. The chain is started automatically as soon as you reach level 25 and will take you to your faction's capital city, where you have to take on a few lore-related puzzles and can then choose one out of three different tasks to complete, before you get to take a final test that requires you to prove your flying abilities. Once you gain access, you can enter the Abyss through one of the portals located at your faction's major quest hubs (for example above Verteron Citadel or Altgard Fortress) - the portals are only reachable via flight and will take you to the main fortress of either the Elyos or the Asmodians in the Lower Abyss.
The Abyss will provide you with more quests and monsters that can take you all the way up to level 50. Although you can choose between staying in your relatively safe homeland to level up, taking the more dangerous route of leveling in the Abyss will reward you with increased experience gain from monsters, as well as a steady income of Abyss Points - which serve as both a measurement of your PvP rank and a currency for PvP items, crafting recipes and Abyss abilities. The highest-ranked officers of a faction get access to the most powerful rewards, some of them can actually transform a player and enhance his power considerably for a short amount of time, granting him additional, devastating abilities. However upon death, or when you buy rewards, you lose Abyss Points, and can even be demoted to a lower rank again.
The drawback of hunting in the Abyss is of course the ever-present danger of being attacked by the opposing faction, and the designers have done an excellent job at creating points of interest and choke points in the Abyss to bring them together. They consist of three layers:
The Lower Abyss is where you start out and where you will find the most faction NPCs and quest givers.
The Upper Abyss contains higher level mobs, more fortresses and artifacts, and can be reached by taking one of two teleporters that are located at the opposing ends of the Lower Abyss.
At the center of the Upper Abyss is the Eye of Eleshuranta, where you can fly through to reach the Core of the Abyss - located there is the biggest fortress, as well as nine artifacts.
Both factions have their own stronghold, which is the only place in the Abyss where you are safe, and where you can find quest NPCs and all the vendors you need, including trade brokers and a warehouse. There are also several portals that can take you to the most important regions of your home world or to other fortresses in the Abyss if your faction has control over them. These fortresses are - just like the artifacts - initially occupied by the Balaur and in order to conquer them, your faction has to defeat the numerous Balaur elite NPCs, overcome the fortress gate and then defeat its Guardian, a raid boss of considerable power depending on the fortress you are attempting to take. Naturally you will also have to battle the other faction for these fortresses, creating what the designers call a PvPvE environment.
Sieges are only possible once every few hours, and after the siege window is over, ownership will be locked for a period of time. Owning these fortresses grants your guild and your faction bonuses, like being able to teleport into it, access to its fortress-specific PvE-dungeon, and influencing the tax rate and prices at NPC vendors.
Although flight plays a very important role in the Abyss, and most PvP encounters will involve a lot of aerial combat, a fortress is protected by a force field that makes flight in its immediate area impossible. To conquer a fortress, legions will have to defeat the opposing troops on the ground and use siege weapons to gain entry to the fortress. If the fortress is owned by a player faction, the defenders will have to fortify the gate in order to hold it long enough to drive back the attackers.
Additionally, you can take over artifacts that are usually located near a fortress. Artifacts will grant your faction bonuses like buffs, or they provide the ability to debuff enemies or even activate a powerful magic area attack. Similar to a fortress, you must first defeat the artifact's Guardian to be able to use it. Once you have conquered an artifact, friendly NPCs will spawn and guard it from then on.
Once all fortresses are taken by one of the two factions, the battle against the Balaur is far from over. It is uncertain what exactly triggers Balaur attacks, but they supposedly serve to help balance the power struggle between the Elyos and the Asmodians and thus prefer to attack a fortress of the stronger faction. When the Balaur attack, they do this by deploying their powerful Dreadgion - a battleship that can easily wipe out whole armies with its arsenal of weapons. It also drops a horde of Balaur soldiers onto the battlefield.
Rumor has it that this battleship is actually a raid boss, but it has yet to be defeated on the asian realms, and it is not sure if it is actually possible to do it at this time. So like the in-game tutorial says, whenever a Dreadgion appears it is better to take cover.