The Suppression Device is a large cannon built by the Combine and is located on top of the Overwatch Nexus. It fires some sort of super-heated plasma beam into the sky which then hits the ground and vaporizes anything that's unfortunate enough to be close.
The Suppression Device is powered by three combine generators within the Overwatch Nexus. The three generators also powers several force fields around the Nexus.
When the Device was shut down by Gordon Freeman under the Street War, reinforcements could come through and pushed back the Combine.
Trivia
The Suppression device appears in the map d3_c17_10a and when it's disabled in d3_c17_11.
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Suppression Device
