Note: This article is not up-to-date with the latest ingame info, still have to update it.
How to access/reach the Abyss
Once your character turns lvl 25 you'll gain the Abyss Entry Quest (AS,EL) you have to finish the quest chain to be able to use the Abyss gates you can find in
Asmodians: Altgard and Morheim
Elyos: Verteron and Eltnen
The AbyssWhen you zone into the Abyss you'll start at your main faction stronghold in the lower Abyss. For Asmodians its Primum Stronghold at Primum Landing and for Elyos its Teminon Stronghold at Teminon Landing.
This stronghold is a safe zone as its being protected by some insta-death force-field against the other factions. The starter fortress has various NPCs (vendors, bank, etc.), a bind-point Obelisk plus a teleporter to take you back to the normal zones. The Balaur NPC faction inhabits all layers of the Abyss and can attack fortresses and will take part in the Abyss fights. The mechanism and the way the Balaur act are not very well known yet, but it can happen they show up anytime. All of the Abyss is a flight zone and there are certain flight-rings that will enable you to fly longer distances (the quest will explain it).
Like on the first picture the Abyss consists of upper, lower and core part. For better orientation here are maps of the three different zones:
Lower Abyss
Contains 3 fortresses and 8 artifacts
Abyss Core
Contains 1 fortress and 9 artifacts
Upper Abyss
Contains 5 fortresses and 14 artifacts
From the lower Abyss you can reach the upper Abyss part via transportation lightbeam tunnels which you can find at the Eastern and Western Shards of Latesran. Once your faction holds a fortress you can teleport there by means of the normal teleporter at your stronghold as well from what i hear. The Abyss core you can reach through the middle of the upper Abyss.
Abyss Fortresses & artifacts
Inside the Abyss are a total of 31 artifacts. Either of the three factions can hold them. Should you capture an artifact, you can hold it for 24h. Your legion emblem will be displayed on it as well. There are 14 different artifact abilities that can be accessed pending on which artifact you own. To activate artifacts you need Artifact Activation Stones that are crafted by Alchemists. Only Legion Masters can use the artifacts with default legion settings. Artifacts will be guarded by NPCs from the faction that owns them.» More info about artifacts and their various abilities
The nine Abyss fortresses are guarded by an Aetheric force field that prevents assaults from the air. You have to take down the gate (looked rather quick from the videos i saw so far) and then destroy the Aetheric field generator to enable air-assaults. Defending players can operate a Gate Restoration Device (needs Castle Gate Restoration Stone from stronghold vendor) from inside the castle to repair the Gate Guardian Stone and delay the attackers. The Attackers can use siege weapons in their assault to bring down doors faster. From the looks of it a component from every crafting skill is needed to craft a Siege Weapon (cooking brews the Driving Water). There are also Multiple Fire Cannons you can buy from the legion vendor. Some kind of keep guardian NPC boss is present inside the fortress from what i saw. You have to kill him to claim the fortress. Once done, NPCs from your faction will re-populate the fortress. Ownership of the fortress will go to the legion that contributed the most (no idea how the system works, but i guess most Abyss points accumulation during the siege).
Claiming fortresses will yield, beside some other rewards like Abyss points, silver and gold medals you need to buy the higher ranking Abyss gear. I heard that medals are handed out to the top ten (3 Golden Contribution Decoration and 7 Silver Contribution Decoration) people with most damage done. Note: This info is not exactly correct, but you'll get the picture. No one knows the exact amount of medals given out or the formula. Seems to be more then ten tho. The medals can be traded (or sold via broker) and since its damage done, some guilds come up with a system to redistribute them since tanks/healers won't top the list. A rumour i heard said that a legion claiming a fortress would earn some money for their efforts from some kind of taxes the fortress accumulated and keep getting kinah every 12 hours from it. I also read something about a timetable for sieges, which would mean not all forts are attackable at all times but only during certain days (Wednesday, Friday, Saturday and Sunday). The new Abyss Fortress Dungeons implemented with patch 1.5 can only be accessed if your faction owns the fortress.In Aion there are short ingame Abyss Keep Sieging Tutorials, check the link for videos (watch in HD to read text).
Earning Abyss Points & Ranks
Your character info window will show you your current rank, abyss points and recent data about earning & losing them. While you accumulate abyss points your rank will increase, should you lose/spend Abyss points your rank will decrease. They also act as a currency for buying Abyss gear with them. Here is a table, that i found (1,2), which will explain the system a bit more:
Grade 9 Soldier is the lowest rank and Supreme Commander the highest. The column "Required Positions" is labeled a bit misleading, it shows the maximum number of players that can have this rank. There will only be one Supreme Commander for example. The sorting inside the ranks is done by total amount of Abyss points the players currently have. If eg. there are more players with enough points to have a certain rank, but fewer spots per table, only those with the highest amount of points will have that rank. The others will be "downgraded" a rank.
The table does not reflect the whole abyss point system, since eg. you'll get less points should you kill a player that only has half HP, but should be close enough to give you an idea about it. "Points Gained" are points you would get by killing a player with that rank. "Points Lost" refers to the points you'll loose at a certain rank being killed. Looking at the table makes it obvious that in the beginning (with lower ranks) you don't lose that many points, but the higher your Abyss rank, the more points you will lose should someone kill you. I'm not sure if the same system (if you kill a NPC five levels lower then your level in Aion you won't get XP/loot) applies for Abyss points as well, so if eg. you kill someone 10 ranks lower then you you'd get nothing. There is a larger discussion about how your playstyle will change depending on Abyss rank. Here is a translated interview with a high ranking korean player. Basicly he describes how his playstyle changed and the higher his rank, the more careful he had to become only to pick fights on his terms. From the discussion there are also players that claim the best route at a certain point is to just grind NPCs and do repetitive quests inside the Abyss to gain Abyss points, since its "safer". Getting killed at very high Abyss rank will result in losing AP worth a few hours grind. Time will tell how this system works out and how it will change.
What is the motivation to keep rank and not cash everything in immediately? Well, some of the gold Abyss gear is very costly in abyss points and you need to save quite a while for them. Beside the prestige of keeping a high rank, each player with rank 5 Star Officer or higher will get access to a special Abyss transformation skill that will enable you to transform into a powerful avatar for around 10min (cooldown is two hours). Each rank higher up you gain more special skills you can use while in this transformed state. You need to buy ingredients with Abyss points to use the transformation. For a 5 Star Officer it costs 3000AP to transform, each higher rank costs 500AP more.» More info about the Abyss transformation abilities.
You earn Abyss Points (AP) via killing the other faction (players & NPCs), killing Balaur & other Abyss NPCs, claiming an Abyss fortress and from quests. Even before lvl 25 there are a few quests that will give you abyss points. I also heard rumours about some rare quest you can get while crafting that will yield 10.000K and 700 AP. You lose Abyss Points by getting killed inside the Abyss and by the other faction or by spending the points on gear. The gear you buy with Abyss points has no rank requirement to wear it. So once you bought it, you'll be able to use it.
The dilemma to solve for many players will be: When to cash in your Abyss points? Of course the higher lvl abyss items are much better, but maybe cashing in early will give you an advantage? I know some players are eyeing boots with a speed boost (important for PvP) first and Abyss points seem to be the first reliable way to obtain them early on. As an example: The Guardian Squad Leader's Plate Boots are lvl 30, blue crafted boots are lvl 33 (tho expensive ingredients required and an armorsmith master (rank 400)) and blue quest rewards are lvl 40 (AS, EL). Maybe you'll get lucky with a world drop, but no way to count on that. Saving points will also depend a bit on your skill and playstyle. You also have to take into account that the high lvl Abyss items need medals to pay for as well. Personally i like the idea to use PvP points as a currency and you have to think about how/when to spend them.
Hope this could shine some light on the Abyss, please correct me if you spot any errors in the report.
Those of you that played WAR will remember the ton of grey items you could get via quests that were supposed to have a hidden purpose (but had none). While questing in the asmodian starter zone i encountered the 
With the war against the Mau ending in victory for the Asmodians, the Five Shedim Lords declared that a permanent arena should be built upon the site where the Legions had encamped in Altgard. They believed all brave Asmodian warriors would benefit from training in dueling with the sword. As many master artisans, architects and other famous Daevas of the time participated in building Impetusium, the construction progressed rapidly. The Arena, decreed Pandaemonium, was to be built in a 'grand and classical style.' Even in the ruins, the outlines of this style can still be traced. With the construction of Impetusium, sword fighting became a highly popular method of competition. There were duels to settle points of honor, for recreation, for cash prizes, even to settle marriage suits. Daevas came to the arena in huge numbers from all over Asmodae to duel with one another. In the broader context of Atreian history, dating back to time immemorial, Impetusium lasted only a few fleeting moments. Nonetheless, in that time it produced numerous heroes who enjoyed the praise and adoration of the public.
The name 'Shaga' will be instantly familiar to anyone who has studied the history of Impetusium to even the smallest degree. Shaga arrived at the Arena seemingly out of nowhere, and became a legendary hero. Instantly recognizable in his black helmet and black armor, a solitary red feather on the breastplate, he had no peer among all Impetusium's fighters. The ringside poets said of him that he moved as lightly as breeze-blown apple blossom, and struck with the sudden ferocity of a summer storm. Shaga's legend was not confined to the Arena. A true wandering hero, it was said he would appear whenever and wherever people needed help. He never spoke, but instead greeted people with his gentle gaze. Many women who glimpsed those eyes through his visor were smitten, and became sick with love. It was not until much, much later that the most astonishing fact about Shaga was revealed. He was not a he at all, but a she.
Those peaceful and festive days in which Impetusium thrived did not last for long. The discovery of Elysea beyond the Abyss marked the end of peace. The Shedim Lords assigned all Asmodian Daevas to fight the Abyss War. With Altgard no longer a main rallying point, it was decided that Impetusium should be closed down. Although a few Daevas objected to the decision, the majority understood that the dark clouds of the Elyos-Asmodian War had cast a chill over the excitement of the great arena. It was no longer a time for sport, but for battle.
Since i decided to join the battle between Elyos, Balaur and Asmodians i thought its time to create a 
