Item from Half-Life 2 by Valve
The heart of the Citadel — a contained sphere of dark energy that powers the Combine's Earth operations and nearly tears a hole between dimensions when it ruptures.
The dark energy core is the Citadel's power source, communication relay, and — when it destabilizes — the most dangerous object on Earth. It powers the Combine's operations, maintains communication with the Overworld, and when Gordon disrupts it at the end of Half-Life 2, it begins an uncontrolled meltdown that threatens to destroy City 17. Through Episode One, Alyx and Gordon descend into the collapsing Citadel to stabilize the core enough to buy time for evacuation. They partially succeed — containing the explosion long enough for the city to evacuate before the core finally detonates, creating the superportal that Episode Two's rocket mission must close. The core represents the Combine's fundamental vulnerability on Earth — all their infrastructure depends on a single power source, and that source is a barely-contained dimensional rift.
A swirling, luminous sphere of contained dark energy suspended in the core chamber of the Citadel. The energy shifts between deep blue and blinding white, contained by Combine field generators arranged in a circular pattern. The chamber itself is massive — a vertical shaft with the core floating at center, observation platforms around the perimeter, and conduits carrying energy throughout the Citadel. When the containment begins to fail, the core pulses erratically and the surrounding structure groans.
Also known as: Dark Energy Core, The Core, Citadel Core, Combine Reactor