Chain attacks do many things to help Aion's combat move smoothly and efficiently. They clean up the UI, they offer flexible choices, they keep the combat looking fluid, and they're a lot of fun to pull off.
I can finally see my screen!
Chain combat is exactly as it sounds -- it's a series of skills that must be used in a certain order before the next one is available. Skill 1 leads into skill 2, skill 2 leads into skill 3 and so forth. On the face of things, this sounds pretty mundane. Aion isn't exactly the first game to make people use skills in a certain order.
What Aion does do to make the concept fresh and unique is making use of the UI beyond the skill bar. In most other MMOs, you would have to put each skill separately onto the skill bar and then choose to execute them in order. While that's easy to do, it sadly takes up space on the UI and may force you to add more tool bars to clutter up the screen.
Aion's approach is more dynamic than manually slotting every single skill. When you use a skill that begins a chain, the next skill in the chain appears next to your character and the original skill is replaced by the upcoming skill in your hot bar. So, if you use skill 1, skill 2 will now appear next to your character and skill 2 will temporarily take up skill 1's slot in the hotbar.
Choices, choices, choices.
The chain system also pushes the user to make choices in combat. Chains aren't always choice 1, then choice 2, then choice 3. Sometimes a chain can branch, leading the player to two different paths of options.
The options can sometimes terminate the chain early, forcing you to wait for cooldowns to expire before you attempt it again, or keep the chain going at the expense of missing out on a skill you may have needed in the other branch.
Take, for example, the Gladiator's skill chain at level 13. Pressing your chain button constantly will lead your Gladiator to three different types of hard one hit attacks, but if you perform the first attack and then look at the chain's options, you'll see an ability which provokes the enemy and forms a damage shield around your character. Choosing that shield ends the chain, but it absorbs the damage you're about to be dealt. So, the question becomes, do you want to make the fight last longer and take less damage, or end the fight faster and possibly take more?
Smooth animations for smooth chains
Setting the attacks up in this manner also allows the developer to know exactly what abilities will precede and follow others, letting them set up the animations to make combat look fluid.
The Gladiator's two strike chain makes the first attack hit the enemy from the left, but the second skill in the chain utilizes the momentum from the first to attack the enemy from the opposite side, just like a person would if they were swinging the weapon.
Combat looks more natural, all because the developer can anticipate a player's actions instead of watching as the player mashes buttons, canceling out animations that may only be halfway completed before a new animation moves in.
Chains: They do more than you expect
One single game design option that seems simple from its description has a substantial impact on how the player perceives combat from their UI, graphics, and tactical choices. This option really does offer more flavor to Aion's combat, and it's one of the reasons I'm still hanging on and playing Aion.