Ar tonelico Qoga: Knell of Ar Ciel is set two years after the happenings of Ar tonelico II: Melody of Metafalica and takes place in the Sol Cluster. The player controls Aoto who witnesses the murder of an old man by Clustanian soldiers, the faction in power of the region. A girl named Saki survives the assault and turns out to be a Reyvateil, basically the game’s version of magically gifted persons. Now Aoto takes care of her, flees from the hostile soldiers and tries to find out the reasons for their hostility. Of course the plot gets more involved during the game and is mostly told through dialogue and occasionally animated cutscenes. Like the predecessors, the game features a lot of quirky situations, suggestive dialogue and girls with few clothes.
Much time is spent with exploring the environments which are confined at first but get more open later in the game. Here there are treasure chests with better equipment to find, dungeons to visit and randomly started battles to fight. Those fights take place on a separate screen and because they are real-time based, only one of the up to three party members are directly controlled by the player. However, the game is paused when the combat menu is opened which allows to perform special attacks, use items or switch to another character. The speciality of the combat systems are the Reyvateil which is always an additional member of the fighting party. She is defenseless (and therefore has to be protected) and sings during combat. When the player attacks in rhythm to the tune (i.e. during a peak), the damage increases. Additionally the player can start a purge and has to solve a mini game (holding buttons and shaking the controller); after success the Reyvateil loses part of her clothes and gives positive effects to the whole party, failure has negative effects.
The second major part of gameplay is the interaction with the Reyvateil party members. Besides decisions made during the story (which may have significant consequences on its flow), the player can initiate dialogue talk about various topics. While conversation is possible with all party members, it is important to build a connection with the Reyvateil so the player can dive into her Cosmophere (mind) and has to deal with her deeper psychological problems. This mostly plays out with dialogue, but there are also some special sequences with other mechanics. The payoff are Hyuma, different personalities which can be “equipped” for a changed combat melody and other effects after using purges in combat.