Caleb-Omicron-27 is the Alpha of Vanguard Cohort Omega-21, or more informally identified as the Silent Guardians. The Cohort was created initially as an escort for Inquisitor Drogan (see art.66120861449/Inquisition) during his experimentation on xenos specimens in late M41. However, their guard duty came to an abrupt halt when Graia was invaded by Waaagh! Grimskul (see art.10093687/Grimskul), and an Ork Rok separated the Inquisitor from the rest of Omega-21. Seeking retribution for their failure as bodyguards, Caleb-Omicron-27 personally initiated the Yuga/Sundown protocol for the rest of his Skitarii Vanguard Cohort, proceeding to make their way to the nearest civilian population centre, Hab Andris, to repel the Orks and stop them from claiming valuable organic and non-organic matter.

The Siege of Andris (see art.267848190/Andris), as it is recorded, lasted 3 weeks. Not one Ork penetrated the iron cordon of Skitarii surrounding the Hab Block the terrified civilians were cowering in. Countless greenskin invaders fell to the barrage of Radium Carbines, or to the lethal radiation that plagued the corpses of their brethren. Caleb-Omicron-27 himself routed no less than 3 raiding parties single handedly by eliminating the Nobs leading the charges in brutal single combat with his broken power sword.

Eventually, attrition was wearing down the squad. Without their regular cleansing routines from their own Radium Carbines, the Cohort was becoming weaker, their combat protocols becoming sluggish to respond. Benjamin-Omicron-43 was the first to fall, torn limb from limb by Orks scaling the Hab walls. The intense radiation emanating from his body claimed their lives too, pleasing the dying man-machine when his emotional inhibitors failed. Two more Silent Guardians fell, Ajax-Omicron-12 and Lydia-Omicron-88, before aid arrived, in the form of Chaos Space Marines, of the Chosen of Nemeroth warband (see art.72539901010/Nemeroth).

The Skitarii retreated briefly with methodical finesse as the two forces clashed. The Renegades won out, and turned their eyes to the Silent Guardians. The fighting was brutal and bloody. Two more Skitarii were rendered irreparable, and two more severely damaged. By the time the Space Marines of the Blood Ravens Chapter (see art.26193356711/BloodRavens) had arrived to assist the conflict, the last warriors of the Cohort stood silent sentinel over a huddled group of civilians, several dozen Ork corpses and the cold shells of ten Chaos Space Marines. The Sergeant in command of the Blood Ravens awarded Caleb-Omicron-27 with the right to wear his own Blood Ravens heraldry and gave him his own power sword, to replace Caleb's damaged one, before swiftly leaving with the Chaos dead. The civilians were rehabilitated, but all non-Mechanicus who endured the Siege died soon after due to **redacted**. The irradiated Skitarii were later decontaminated when the rest of the AdMech forces arrived to collect their warriors. However, it was noted that the Silent Guardians have possessed a slightly increased resistance of about 2% to radiation poisoning, allowing them to operate longer without cleansing.

The Cohort have since performed exceptionally well in all encounters against Orks, Chaos Space Marines, or other Vanguard Skitarii in combat simulation.