From Command and Conquer Wiki

The Relay Node (also known as the 'Control Node') was a massive structure constructed by the Scrin on Earth at Ground Zero that was the key element of their invasion. Its destruction by GDI ended the Third Tiberium War.

The Relay Node was constructed during the initial stages of the invasion, while the majority of Scrin forces launched a diversionary attack against major urban centers in Blue Zones and several Yellow Zones. With its completion, the Scrin forces could safely deploy the bulk of their military on Earth, as the Tiberium-based radiation it emitted ensured the stability and continuous powering of their organisms/machines; the Relay Node was also discovered to have a massive range, great enough to reach the Moon. After the construction was finished, most Scrin forces withdrew to Red Zones to erect the massive 'Gateway' towers. The Relay Node appeared to be the main hub of a system, and hence an Achilles' heel of sorts.

Before all the Gateways could be completed, the Relay Node was attacked and obliterated by GDI forces in a daring assault. Despite massive resistance and fielding of their most powerful units (including the Mothership), not to mention an inconvenient Nod presence in the region, the Scrin were defeated and the Relay Node destroyed. With its destruction, most Scrin infantry literally dissolved and their aircraft, "vehicles" and buildings came crashing down. Research of the remains of the structure still continues.