As the name suggests, a Deacon serves primarily for morale and religious purposes rather than combat. The rank of Deacon only exists within the Ministry of Tranquility, and is the lowest rank available. A Deacon is sent to help reinforce the spirituality of ships in which little is present. They are taught in the religion of the Covenant and work to increase faith in others. Deacons are also learned in the "Sacred Glyphs" that the Forerunners use in their machines.Deacon is the only title in the Ministry of Tranquility that is open to Unggoy, likely making the rank the only governmental position that Unggoy can hold. Deacon is also one of the few jobs Unggoy can get that does not involve hard manual labor or military service. It is noted that Unggoy Deacons are quite rare due to the intelligence that the job requires - whether other races can also hold the title is uncertain. Unggoy Deacons command more respect than the rank and file Grunts, but are still held in low esteem; for example, the Kig-Yar Shipmistress Chur'R-Yar once nearly ordered that a Deacon named Dadab to be pushed out of an airlock, despite the Unggoy being an official Ministry representative, and only restrained herself from doing so due to the Unggoy's ability to communicate with the Huragok Lighter Than Some. Unggoy Deacons do not command as much authority as even a Kig-Yar Minor, and so it is unlikely that they minister to any of the other races.
Another duty given to Unggoy Deacons is to ensure that the Kig-Yar crews of the Ministry missionary ships obey Ministry laws and regulation. Given their low rank, the piratical nature of the Kig-Yar, and the contempt in which Kig-Yar hold Unggoy, this is a largely hopeless endeavor.
- Deacons are mentioned in Halo: The Flood, Halo: Contact Harvest, and Halo: The Cole Protocol. The Unggoy Dadab and Pipit are the only known named Deacons. However, there are presumed to be many more, due to Dadab mentioning the group that he had studied with to gain the title.
- It is added that a Grunt becoming a Deacon is very rare, presumably due to a Grunt's low intelligence and sometimes insincere belief in the "Great Journey." Therefore, it is odd that Deacons are included in Halo Wars as upgrades for Grunt squads. They "inspire" the other Grunts, resulting in the lance inflicting higher damage. However, because there is one per lance, it contradicts the prior rarity of Grunts becoming Deacons, meaning that 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).