In Ar Tonelico Qoga, in the glossary under the Necrohouse of Ayatane, it talks about that some of the Teru Clans began to hate humans after something called the Seven Blood Stains Incident that was caused by Sol Ciel. It was this event that led to the Necrohouse allowing Garthnode Infiria to occur hoping that humans would destroy themselves.
But I don't know what this incident is. The only incident involving the Teru Tribe in Sol Ciel I am aware of was when Bourd kidnapped Misha from the Crescent Chronicle resulting in a battle between the Teru Tribe and Tenba which resulted in the destruction of Aurica's hometown Skuwat. But this was way way way after Garthnode Infiria so this can't be it.
The way it's worded seems as if this Seven Blood Stains incident is supposed to be matter of fact for players of the previous game (since it also clarifies that Mir's Son Ayatane is unrelated to the Necrohouse of Ayatane). So I am wondering where was the Seven Blood Stains Incident detailed? Was it in a drama CD or manga?