STEP FOUR: INVASION

The Gravak launch an invasion of all the rings. As bronze-age masters, the Gravak have the technological superiority (think Ancient Egyptian era). Each race is under threat and must repel, escape, or survive these horrific task masters. 
The Gravak take slaves, will eat most other races, and show no mercy. The Gravak can either be defeated in epic battle, or pushed out of your races lands. Alternatively your race could bend the knee and become a slave race to the Graviak. 
After the invasion your race advances into the bronze age, gain writing - the Gravak use writing to keep track of how many slaves they've taken. 
Describe first contact, a major battle, and how the Gravak finally left.