Ar Ciel's Demise (アルシエルの終焉, arushieru no shuuen?, lit. "The Death of Ar Ciel") is the name of the event that will end happening at the end of the three Ar tonelico games if the True Endings for either Melody of Elemia and Knell of Ar Ciel weren't attained.
This event will happen around 3775 AD, in which the Planet Ar Ciel won't be able to continue sustaining life or even its own existence due to the lack of a Heart of the Land and the Dynamic D-Waves it needs to do so. Once the remaining Symphonic Power (Dynamic D-Waves) the Planet has at its disposition has been depleted, the Planetary Core and the Wills of Ar Ciel that protected it will die off, causing all of the natural forces the planet created and maintained, such as gravity, to vanish away.
The vanishing of gravity will complety disrupt the structure of the Planet, which will turn into a massive clump of rocks that will expand and separate from each other at such high speeds that any person looking at them would think that it was a swarm of rocks rising from the ground. After this, the lava and liquid metal that formed the planet's core will shoot up back to the space, and most likely, all living creatures that were in the surface won't be able to survive both the launching of the rocks and this. Even if they managed to survive these, the atmosphere wouldn't exist anymore, which would cause all the living beings to die from asphyxia and due to exposition to the radiation present in space, thus meaning the total annihilation for everything that once lived in Ar Ciel.
Some could be thinking that the inhabitants of the Towers and Metafalica would have a chance of surviving due to the Plasma Bell and the Musical Corridors, but this is no more than a vain hope: the Plasma Bell requires Ar Ciel's own gravity to work, similarly to how the Musical Corridors merely exert control over Ar Ciel's atmosphere, and once both have disappeared, they will become unable to accomplish their functions. Finally, Metafalica does have more hopes of surviving because the continent itself and everything it needs is created from the feelings of the IPDs, so they could instantly generate an atmosphere and gravitational field if it were needed. However, this would entirely depend on the mental fortitude of the IPDs, and it is uncertain that they would be able to create an atmosphere resilient enough to withstand the cosmic rays from the space.