From Halopedia, the Halo wiki
Alo Sebukah was a San'Shyuum politician, holding office as vice minister of the Covenant's Ministry of Abnegation. Following the outbreak of the Great Schism and the Covenant's collapse, Sebukah was captured by the Office of Naval Intelligence and imprisoned at Midnight Facility, registered as Internee 689-167-53 and identified by ONI agents as Subject DENVER.
Alo Sebukah was born on December 2, 2454 to the Sebalim'tet lineage on High Charity at the Terrace of Illumination, Third Cloister. He eventually rose to the position of vice minister in the Covenant's Ministry of Abnegation.
With the outbreak of the Great Schism and the Flood's attack on High Charity, Sebukah tried to escape the Covenant holy city, fleeing for a harbor. Before he passed the last checkpoint, a nearby terminal activated. When Sebukah approached the terminal, he heard the "pained" voice of the human smart AI Cortana, who had been captured by the Gravemind within High Charity. Upon reaching the harbor, Sebukah eventually escaped High Charity in a damaged skiff. With the skiff's systems failing and minimal temperature control, he was forced to ration his supplies for weeks.
Imprisonment at the Midnight Facility
By the time the corvette UNSC Coral Sea discovered the skiff amongst a cluster of debris in orbit around Substance, Sebukah was just barely alive. Coral Sea later brought Sebukah to the Office of Naval Intelligence's Midnight Facility. Now referred to as "Subject DENVER", only a handful of ONI agents knew of Sebukah's existence, including Zed Rhodros.
Sebukah was imprisoned in Cell 56 of Midnight Facility's Lambda Block. On January 15, 2553, Sebukah was interrogated by ONI agent Oberon 5. He recalled his encounter with Cortana, speculating that she had encountered the Flood, and grimly noted the Flood's ability to corrupt AIs and what it meant for humanity. In 2558, this interrogation would later be reviewed by Fireteam Apollo.