The Slipspace core was a crystal used by the Forerunners to create slipspace drives for their ships. When a new drive was needed, a flake was chipped off from the original crystal and used as the base of the drive. It was said that only the Master Builder knew the location of the core. The exotic composition and slipspace interactions of the crystal were incomprehensible to even the Forerunners, and never replicated.
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