Knights Cenobium are incredibly special and elite warriors of the Inner Circle of the Dark Angels during the Great Crusade and Horus Heresy. If you thought the Deathwings were elite, the Knights Cenobium are the elite of the elites. Think of them as the Companions of the Adeptus Custodes for better context.
Within the First Legion, there were many separate Orders, each dedicated to a singular method of war. The Cenobium 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’s put together using a cell of warriors from an Order of the Dark Angels on the eve of a mission that requires their unique skills. They wore Cataphractii Pattern Terminator Armour and wielded Terranic Greatswords and Plasma-Casters. It is unknown what has happened to these guys as they were never stated to be disbanded nor dissolved.
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.