The Rebellion can refer to the Resistance in Kowal against the Uthgard Empire, or it can mean the Rebellion of mages in Ish.
In Kowal
A group of rebels has formed a resistance in Kowal to fight against the Uthgard Empire. They are led by people such as Arnexi Zadeku, Sundaiya Kotas, or Jorin Hal.
In Ish
The Rebellion in Ish is a rather new group of mages who oppose the rule of the Emperor and the Wizards of Ish. They are hunted mercilessly throughout the Empire, but as magic has reawakened, they have grown in numbers and in power.
Secret is their strongest advantage, so very little is known of them.