The World Killer Expansion continues the story of Earth Under Siege: Flashpoint beyond the events of the core game, introducing a new campaign that follows one of two possible endings from the original storyline. As the Xethan invasion escalates, humanity faces new threats, advanced enemy technologies, and a powerful intelligence directing the alien forces from the shadows.

This expansion features a branching campaign with 14 new missions, and during a single playthrough players will experience 8 of them depending on the choices they make. Additional Special Ops missions further expand the narrative and provide new challenges for experienced Operatives. The campaign also introduces outdoor environments, new enemies, and additional content that broadens the scope of the original game.

World Killer Expansion also introduces Level IV progression, allowing Operatives and Headquarters divisions to advance beyond the limits of the core game.

Fully integrated with the core game, this expansion builds upon the stealth, infiltration, sabotage, and tactical combat systems, while significantly expanding its campaign, progression, and gameplay content.