Aegis War

The Aegis War is a cataclysmic war in Xenoblade Chronicles 2. The war involves both Aegises: Malos and Mythra. It is the main focus of Torna ~ The Golden Country and its legacy plays a role in Xenoblade Chronicles 2.

Cause
The initial spark is when Amalthus brought back two Core Crystals, Malos's and Mythra's, to Alrest after climbing the World Tree. Back in Indol, he awakens Malos, who breaks from Amalthus's control and proceeds to raze the entire world with his destructive powers. To contest him, Amalthus gave Mythra's Core Crystal to Addam who awakens her as his Blade at the Olethro Ruins.

Torna ~ The Golden Country
During the Aegis War, at least two Titans sank into the Cloud Sea, Coeia being among them, to Malos's rampage. At one point, Malos returns to Amalthus to taunt him and reveals that the Kingdom of Torna is his next target. At some point, the war spread to Genbu, as evidenced by the battleground at Memorial Ruins.

Legacy
Although Malos was defeated and the war ended, it left a huge impact on Alrest; people began to fear the power of the Aegises. Some nations desire that power while others want to destroy it. Amalthus then bars entry to the World Tree due to the war, and enlisted Ophion to guard it from any intruders. The Kingdom of Tantal was formed in the aftermath of the war, claiming to be the descendant of Addam. Jin and Malos form the Torna organization with the intention of obtaining the Aegis to open the way to the World Tree, in order to kill the Architect and destroy Alrest due to the role the war played in the death of Lora.