From Halopedia, the Halo wiki
A Deacon was a low-level office of the Covenant religion and the lowest rank within the empire's ministries, though the only office obtainable by the Unggoy. Organized under the Ministry of Tranquility, Deacons typically served as the ministry's representative on expeditionary ships, ensuring all crew members followed the Covenant's writs and laws.
The office of Deacon was the lowest rank within the Covenant's ministries, but it was the only position open to Unggoy and one of the few occupations an Unggoy could obtain that did not involve hard manual labor or military service. Deacons were expected to understand the Covenant's Holy Writs and help explain these laws to other members of the Covenant. A Deacon was expected to be familiar with every sin, and to have full knowledge of the punishment for every transgression. Deacons also familiarized themselves with the "luminations" depicted by luminaries, as well as their meanings.
A Deacon was attached to most expeditionary elements of the Ministry of Tranquility, often to ensure Kig-Yar crew remained in line, and it was the responsibility of the Deacon to ensure that all Covenant members followed the applicable Writs as they carried out their orders, as well as provide sermons for the crew. However, as the Unggoy garnered little respect within the empire, most Covenant members displayed insolence towards Deacons and ignored their authority. In fact, many Unggoy that held this office had met with "unfortunate accidents" for opposing irreverent members of the Covenant. When a Ministry of Tranquility vessel discovered an unidentified installation or ship, it was the Deacon's duty to supervise the search, or offer a blessing at the very least, for potential Forerunner artifacts. These Unggoy dressed in orange tunics, emblazoned with a yellow, circular symbol.
Deacons also served as spiritual leaders for Unggoy, though this was considered a lofty goal and difficult to obtain. Deacons were able to sponsor pilgrimages to High Charity. Deacons would occasionally serve as translators and intermediaries for Huragok as well, as the role of translator was considered beneath Sangheili for the training took away time from their training and time on the battlefield.
Selection and training
Most Unggoy were not considered qualified for Deaconship, and those that were selected for the office were only chosen due to their intelligence and ability to understand the Covenant's Holy Writs. Those Unggoy selected for Deaconship went through an educational course and training voyages at the Ministry of Tranquility's seminary. Deacons in training were educated by San'Shyuum priests at these seminaries. Candidates for Deaconship shared a dormitory, a crowded warren of low-lit cells deep in the base of the ministry's tower in High Charity.
Behind the scenes
In Christian churches, a deacon is a member of the diaconate, an office that is often associated with religious services of some kind. In some churches, a deacon is a member of the clergy, while in others the deacon is a mere layman.
In Halo Wars, Deacons are available as the final upgrade for an Unggoy. They further bolster and strengthen the abilities of the Unggoy squads. Interestingly, they wear elongated helmets reminiscent of their Sangheili counterparts. They "inspire" the other Unggoy, resulting in the lance inflicting higher damage. However, because there is one per lance, which contradicts the prior rarity of Unggoy becoming Deacons, it is more than likely that this is just to enhance gameplay and is regarded as non-canon (similar to the Spartan-IIs depicted in the game possessing energy shields, despite the fact that Mark IV MJOLNIR armor does not have energy shields).
List of appearances