A Cryptum is a sealed capsule where a high-ranking Promethean would enter self-imposed exile as punishment for failure or dishonor; however, it was often used by those in power as a means with which to silence respected individuals whose punishment otherwise might prove problematic in the eyes of the public. When in use, their Forerunner occupants would enter a state of meditative hibernation known as "timeless xankara". In this state, the Forerunner would have clear access to the Domain and the ability to explore the immense stores of information contained therein. During this time, the Forerunner would become unaware of the physical world around him, and could remain in a state of suspended animation for thousands of years. Cryptums required an extraordinary amount of force to breach once sealed. They denied access to every species except Forerunners; however, after being designated as Reclaimers, humans have been shown to be capable of unsealing Cryptums. The nature of a Cryptum was unknown to most Forerunners, including Bornstellar Makes Eternal Lasting. However, he was aware that the violation of a Cryptum invoked dire consequences, one of which was angering the powerful Prometheans.
Cryptums were not always exclusive to the Prometheans. The long-defunct rate of Theoreticals, absorbed into the Builder rate over a million years BCE, also used Cryptums in a manner similar to the Prometheans. The Theoretical known as Boundless was forced to enter a Cryptum after continuing her studies on Forerunner origins and Path Kethona in defiance of Warrior orders. After being opened a thousand years later, it was discovered that the Cryptum had been faulty, likely due to sabotage, and Boundless had died inside.
 Djamonkin Crater
A Cryptum was discovered in Djamonkin Crater on Erde-Tyrene by the young Forerunner Bornstellar Makes Eternal Lasting, containing the meditating Didact. The Cryptum vessel, colored copper and steel, was ovoid-shaped, over ten meters high and at least as wide. When the millennial seal was unsealed, the Cryptum emitted an intense flash of blue light, which apparently caused anyone in the vicinity to feel as if time had been suspended. As the Cryptum opened, it parted outward in three sections above the middle, revealing its meditating occupant inside.
While the Cryptum preserved the consciousness of the Forerunner inside for an indefinite period, the occupant's physical body would be maintained in a vitrified state, lacking most of its fluids. The desiccation process was conducted prior to entry in a Cryptum by drinking a chemical known as inchukoa, which caused the body to shed most of its fluids via heavy sweating. Once the Forerunner was removed from the Cryptum, the body would be administered an initial supply of electrolytes, nutrients and other essential substances and then placed in a restorative bath where it would continue its recovery. The body's muscle structure would then be regenerated, a process requiring several days.
The Ur-Didact's flagship, Mantle's Approach, was equipped with a personal "combat Cryptum", which possessed many capabilities beyond the conventional one he used during his first exile. The Librarian sealed the Ur-Didact in this Cryptum, which was discovered over 100,000 years later by John-117 and the AI Cortana on the Forerunner Shield World Requiem. This Cryptum was spherical in shape, with a black outer casing broken by a pattern of angular, orange-glowing "cracks." Unlike the one discovered by Bornstellar, this Cryptum had not withered the user, as the Ur-Didact remained as physically fit as he had been when he entered; it is unknown if this was the work of the Cryptum itself or the fact that the Ur-Didact wore his personal armor during hibernation. The Ur-Didact also appeared to be awake in it to some degree, since he was able to perform covert tasks like hinder use of a Cartographer and manipulate radio transmissions.
As a combat Cryptum, this Cryptum doubled as a means of transport, being able to travel at massive speeds or even through slipspace if necessary, as demonstrated when the Ur-Didact escaped Requiem's core in his Cryptum via a slipspace rift. It also had the capability to scan its surrounding environment or data networks in order to gather information. However, it had no weapons and was vulnerable to UNSC ones such as MACs and missiles. This Cryptum was destroyed when the Master Chief detonated a HAVOK nuclear warhead inside the Ur-Didact's flagship, Mantle's Approach which it had joined with.
The millennial seal on the Didact's Cryptum placed on Earth was sealed by two forces known as the "wisdom of Harbou" and "strength of Lang". Who or what Harbou and Lang were is unknown. However, they appear to be a reference to the German actress and author Thea von Harbou and the film producer and screenwriter Fritz Lang, who were married and made several films together.
John-117 facing a reactivated Cryptum.
 List of appearances
- ^ a b Halo 4
- ^ Halo: Cryptum, "Chapter 2"
- ^ Halo: Silentium, pages 67-68
- ^ Halo: Cryptum, page 62-64
- ^ Halo: Silentium, pages 51-55
- ^ Halo: Silentium, page 292
- ^ Halo 4, campaign level Requiem
- ^ Halo 4, campaign level Forerunner
- ^ Halo 4, campaign level Infinity
- ^ Halo: Cryptum, page 61