- “A vault of ages. In search of wisdom, or to flee punishment, a mature rate might choose the path of endless peace. It is allowed only for the most powerful, whose punishment might prove troublesome to the Forerunner hierarchies.”
A Cryptum or Warrior Keep is a sealed capsule where a high-ranking Promethean would enter self-imposed exile as punishment for failure or dishonor. When in use, the occupant would enter a state of meditative hibernation known as "timeless xankara," during which he would become unaware of the physical world around him and could remain in a state of suspended animation for thousands of years.
Cryptums were created by Prometheans and required an extraordinary amount of force to breach once sealed. They denied access to every species except Forerunners and Reclaimer humans. The nature of a Cryptum was unknown to most Forerunners. The violation of a Cryptum invoked dire consequences, one of which was angering the powerful Prometheans.
A Cryptum was discovered in Djamonkin Crater on Erde-Tyrene by the young Forerunner Bornstellar Makes Eternal Lasting, containing the meditating Ur-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 Forerunner's physical body would still deteriorate, albeit at a significantly reduced rate. The body of the Forerunner would shrivel and lose most of its fluids, in a process similar to mummification. Once the Forerunner was removed from the Cryptum, the body would be placed in a restorative bath, and the body's supply of electrolytes, nutrients and other essential substances was thereby replenished. The body's muscle structure would then be regenerated, a process requiring several days.
Another Cryptum, also containing the Ur-Didact, was discovered over 100,000 years later in 2557 by John-117 and the AI Cortana within 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 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 Didact wore his personal armor during hibernation. Within his meditative state inside the Cryptum, the Ur-Didact was capable of consciously entering the Domain, which took on the visualization of a form of library. This allowed him the opportunity to study the ancient ancestral knowledge of the galaxy for centuries on end. However, the Domain was later revealed to be of Precursor origin, its neurophysical construction being obliterated upon activation of the Halo Array. This unfortunate reality resulted in the imprisonment of the Didact's consciousness as well. In total darkness, the Didact could experience nothing other than his own festering madness for eons. Prior to his eventual release, the Didact also appeared to be conscious to the outside world to some degree, and was able to perform covert tasks like hinder the use of a Cartographer and manipulate radio transmissions across Requiem.
In addition to serving as a prison for the Didact, this Cryptum appeared to double as a means of transport, being able to travel at massive speeds or even through Slipspace if necessary, as demonstrated when the 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 heavy armament such as MACs and missiles. This Cryptum was destroyed when John-117 successfully detonated a HAVOK tactical nuclear warhead inside the 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."