If you thought the Deathwing were elite battlefield warriors for every engagement, the Knight Cenobiums were those warriors whose contextual knowledge made them the eminent warriors of the current engagement, but then retired back to the ranks until their knowledge was called upon again.
Within the First Legion, there were many separate Orders called the Hekatonystika or the Hundred Esoteric Arts, each dedicated to a singular, particular method of war, or keepers of lore against a particular target or type of enemy. As much of their knowledge was either forbidden or of such high value, they recruited from within the legion in secret and shared their knowledge sparingly in case of capture by the enemy. Even within the orders, they may have only met each other under the cover of cloak and darkness, such that may not have known who the other members were.
When called upon, or the context of the engagement required, those members of the Orders would put their knowledge to best use, advising the battlefield commander of how best to prosecute wars, or by unlocking their armouries and providing specialist wargear for the situation.
The Cenobiums stood as the greatest example of these Orders, their champions and foremost warriors each a keeper of the secrets held by that Order and a practitioner of the form of war they embodied. Unleashed where the fighting was most intense, the Knights Cenobium were a temporary unit that was put together using a cell of warriors from any Order of the Dark Angels on the eve of a mission that required their unique skills. They wore Cataphractii Pattern Terminator Armour and wielded Terranic Greatswords and Plasma-Casters.
Considering that the Legion was publicly split in the Second Founding by the requirements of the Codex Astartes, it is little wonder that we no longer see Knight Cenobium units on the battlefield. Where before, the Orders members might have numbered from mere handfuls to maybe a couple hundred or so across the entire legion, and even then, it was only those who had passed beyond the Outer Circles and achieved the rank of "Cenobite" who would form these Cenobium squads. Now it is said that fragmentary knowledge of the old Orders now only exists within the Inner Circles of isolated chapters of the Unforgiven, restricting the lore and experience exclusively within the Deathwing and other officers who have their own obligations which overrule any other allegiance or membership. Even if there were sufficient ranking members of a particular Order within a Chapter in order to deploy a Knight Cenobium in such a fashion, the battlefield context required to justify their deployment would be rare indeed.
Furthermore, recent lore stresses that there has been such a considerable loss of knowledge of the old Orders that the Captain of the Dark Angels 5th Company is titled the Master of the Unseen Ritual, whose job it is to collate and archive all of the disparate and fragmentary data of those Orders, some of which might be held in secret within certain successor chapters, or in the hands of the Enemy. Much of which will have been coded and ciphered in the first place to keep secret from other battle brothers as well as their opponents.
In the Horus Heresy tabletop, the Knights Cenobium occupy the elite slot. Their weaponry makes them a really killy unit, what with their Terranic Greatswords and Plasma-Casters being absolutely brutal against even rival TEQs. That Plasma Flame rule ain't to be fucked with. What makes them unique, however, is the fact that you can choose from 7 Orders before the match starts. These Orders give the Knights Cenobium special rules and abilities, making them incredibly flexible and versatile; basically becoming Your Dudes of the 31st Millennium. At 275 points for essentially a five men squad, they are a bit pricey, but again, their flexibility more than makes up for that.