A Warder is a special unit found in the Adeptus Custodes. The most famous of which is the Warders of the Vaults of Rython. The Warders of the Vaults of Rython were a special sub-force of the Custodian Guard during the Great Crusade and Horus Heresy. Notable for their black-colored armor and lack of bling, the Warders were a sequestered and shadowy sub-division of the Custodes which were normally deployed deep below the Imperial Palace on Terra, likely near the Golden Throne. Think of them as the Custodian equivalent of the Secret Service, chaps who lurk in the shadows, looking for any covert operations going on within the palace. Their nature is so secret that not much is known about them other than their job description.
It is unknown whether the Warders are the Ephoroi or is just part of it. As the Ephoroi was also another Custodian shadowy order with little information on it available and specialized in covert operations, counter-intelligence, assassination, and infiltration just like the Warders. It is thought that at the time of the Great Crusade the Officio Assassinorum fell under the purview of the Ephoroi. Perhaps it still does...
It is also possible the Warders are members of the Hetaeron Guard, as the book Watchers of the Throne: The Emperor's Legion describes them as having armor that is scorched black from constantly being in the Emperor's presence.
The Warders played a significant role during the event of the Horus Heresy.
Arascid Nassau, Horus Heresy and the Fall of Prospero
Arascid Nassau is a Custodian Guard about which nothing appears in the available historical records until his inclusion in the Prospera Censure Force. It therefore remains unknown as to whether Nassau was in fact newly inducted into the Legio or whether his service, perhaps centuries long, had simply been carried out far from the gaze of history.
Several Sodalities of the Warders of the Vaults of Rython were deployed as part of the Censure Force's reserve second wave behind the lead elements commanded by Valdor. Vitally, the Warder Sodalities resisted the circling counterattacks made by the Thousand Sons forces during the Battle for Tizca's cataclysmic final hours, holding back their psychically-augmented onslaughts when the merely human troops defending the Imperium's supply lines and landing zones broke and were slaughtered around them. Arascid Nassau entered the records here in particular for single-handedly dispatching an attacking wave of Prosperine civilians driven insane and twisted beyond recognition by unleashed warp forces, at one stage creating a break-wall of writhing bodies from their dead, which he then defended against alone for another hour of continuous attack.
It is unknown what happened to Arascid Nassau, whether he continues to live up to the 41st millennium in the shadows or died on Prospero in a heroic last stand is unknown. Less is even known to the Warders especially those from the Vaults of Rython. Chances are is that the remaining Warders continue to keep an eye to the High Lords of Terra so they won't abuse too much of their power, lest they want a repeat of The Beheading or the schism of Goge Vandire.
|Forces of the Adeptus Custodes|
|Command:||Companions/Hetaeron - Shield-Captain|
|Troops:|| Allarus Custodians - Aquilon Terminator - Custodian Guard |
Custodian Warden - Sagittarum Guard - Sentinel Guard
Vexilus Praetor - Venatari - Warder
|Walkers:|| Contemptor-Galatus Dreadnought - Telemon Heavy Dreadnought |
Venerable Contemptor Dreadnought
|Transports:||Coronus Grav Carrier - Drop Pod - Grav-Rhino - Rhino|
|Vehicles:|| Caladius Grav-Tank - Dawneagle Jetbike - Gyrfalcon Pattern Jetbike |
Pallas Grav-Attack - Venerable Land Raider
|Flyers:||Orion Gunship - Stormbird|
|Titans:||Warlord-Sinister Pattern Battle Psi-Titan|
|Auxiliaries:||Sisters of Silence|