The story in The Age of Decadence explores several themes: the dangers of a post-apocalyptic world, the thin line between historical fact and fiction, and the competition among political factions.
According to the Imperial Scrolls, the wicked kingdom of Q'antaar and the Empire were locked in a bloody magical war that devastated much of the known world. Each side called upon powers both arcane and divine, summoning Gods and other beings of immense power, whose contest ultimately destroyed both sides. Cities of the once glorious Empire lay in ruin, and bodies of the dead were strewn across the land. Centuries later, the world still suffers from the shadow of civilization's collapse. Where there once was unity, there are now factions struggling for dominance in the ruined world. Where there was once knowledge, there is ignorance of both science and magic. Where there was once chivalry and honor, there is only pettiness and betrayal. And it is in this uneasy divided land of myth and fable that the player discovers an ancient map offering a hint to the Empire's true past.
The player's character has an active role in shaping the plot of AoD, which is nonlinear. The world reacts dynamically to the player's actions. Players can choose from among 7 factions, play them off against each other, or even reject them all. The game is purported to have seven endings, only two of which involve mortal combat.
Players choose from 8 backgrounds/professions at the start of the game. These range from the honor-bound knight to the crafty grifter. Players can also choose, alternatively, to start with no background at all. Different backgrounds result in very different gameplay styles. A background determines a character's initial relationship to various factions. A thief, for example, starts as a member of the Thieves Guild and gets reputation bonuses with other thieves but reputation penalties with enemies. Backgrounds are not classes. They do not restrict or guide a character's future attributes or skills. The Age of Decadence uses a skill-based system, not a class-based system. Characters do not gain levels, but they gain skill points by completing quests or defeating enemies.
AoD supports some customization. Players may choose their appearance (e.g. skin color and hair style) and gender. The choice of gender affects gameplay. Some quests are only available to males or females, and character can receive bonuses to some skills depending on their gender (e.g. males get bonuses to intimidate.)
Characters in AoD have several primary stats: strength (affects damage), dexterity (affects action points), constitution (affects hitpoints), perception (affects chance to hit), intelligence (affects skill point bonuses), and charisma (affects NPC reactions). Stats range from 4 to 10 and characters who achieve 10 can gain bonus traits.
AoD features 23 distinct skills. These range from those pertaining to combat (e.g. Critical Strike) to those that meet miscellaneous needs (e.g. Disguise and Lore).
AoD is purported to have a detailed crafting and alchemy system. These skills can be used to melt old items and forge new ones, poison weapons, create acid, etc.
Combat in AoD is turn-based and focuses heavily on tactical options. The options available partially depend on the attributes, skills, and equipment of the character. For instance, hammers allow a character to knock down enemies, axes can split shields, swords can disarm opponents, daggers ignore armor, and so on. Characters can aim for specific body parts.
AoD does not have party-based combat, i.e. unlike Arcanum for instance the main character does not have any "followers." However, in some situations a character will receive aid from guards, from members of his own faction, etc. These NPCs are controlled by the game AI and by scripts, not the player.
Combat in AoD is realistic in difficulty-- even the best fighters are only capable of taking on a few people at the same time. Furthermore, healing potions cannot be used to recover hitpoints during combat.
AoD features extensive dialog trees. Certain dialog options require skill checks. In AoD, these skill checks are against set numbers. There is no rolling.
There will be allegedly more than 100 quests in AoD and they will have multiple solutions, including options for pacifism and diplomacy. Each quest can be handled in a variety of ways, depending on the character's skills, reputation, and connections to different factions. Furthermore, different quest solutions have different consequences, some of which will be dramatic.
In AoD, there are no plot-critical NPCs. There is always more than one way to acquire plot-critical information. Thus, unlike in some other games, players are not restricted from killing (or at least attempting to kill) certain 'special' NPCs.