From Halopedia, the Halo wiki
Relic cores, also referred to as shrine-cores, are a kind of Forerunner technology generally used for the purposes of power generation. During its reign, the Covenant's relic-hunters and recovery teams located and secured many such cores for repurposing - usually installed into their warships to augment (or even replace) the ship's existing pinch fusion reactors. The use of an irreplaceable shrine-core was the only instance in which a Covenant ship may forgo the use of pinch fusion, using cores extracted directly from Forerunner starships.
The installing of relic cores is a process generally undertaken by Huragok engineers, as the inner workings of the cores' functions remain little-understood. The use of such cores was often inefficient and/or unsafe, but the Covenant viewed the recovery and repurposing of such relics as a high honour, and thus endeavoured to make use of them whenever possible.
Covenant vessels making use of Forerunner technology in their construction may be referred to as "examiners". The Varric-pattern heavy cruiser is equipped with an array of shrine-core power plants to provide energy to its control and weapon systems. In the three-letter transliteration codes used by United Nations Space Command analysts, vessels equipped with a relic core are given an O letter in the code - designating the vessel as Ordained.
The Rasus-pattern interdictor is built around an array of relic cores, referred to as weapon-cores, and used for the purposes of powering the ship's vast array of plasma lances. Due to the irreplaceable nature of the cores, these ships are impossible to replicate without a new trove of cores being discovered, meaning that their loss is a significant blow to the Covenant.