From Halopedia, the Halo wiki
An Encephalon is a piece of Forerunner technology and a vital component of a Monitor. It contains, among other things, the routines needed to summon a Halo Ring's Reformation Spires. Like an activation index, an encephalon can be stored in digital format.
On May 28, 2560, the Harbinger, who desired to restore Zeta Halo's Silent Auditorium, forcibly extracted Despondent Pyre's encephalon. She subsequently used the encephalon to summon the Spires needed to carry out the Reformation. The Weapon later stole the encephalon from the Harbinger and used it to shut down all the Reformation Spires.
Encephalon is another word for brain.
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