The Easter Rebellion was a terrorist cell in the mid 21st century, based in Eastern Europe and headed by a man named Easter. The group had no single political objective, being composed of members of differing philosophies, but united in their disdain for the status of 21st century society.
The Easter Rebellion was a rival of the terror group headed by Racher, though the two did on occasion team up in pursuit of common, anti-governmental goals.
The Easter Rebellion dissolved after Easter died in an Antarctic blizzard during an abortive campaign, and other key members were captured and "re-educated" by authorities. (TOS novel: Strangers from the Sky)