Armaghast is a world located within the WorldWeb of the Hegemony of Man, approximately six hundred light-years from Old Earth, and within one light-year from Pacem and Svoboda.
In the moments leading up to the perceived Ouster attack upon the Hegemony, Armaghast suffered terrorist attacks from Glennon-Height loyalists, who had been quiet for three-quarters of a century. 
During the Web Era, it was governed by a hierarchy of padishas - this maybe suggests at least some of the planet's population to be of Persian descend.
Armaghast contains the largest history of nonhuman artifacts known to the Hegemony - so much so that the world can be considered a "tourist attraction" by serious archaeologists.
Father Paul Dure falsified evidence during a dig on Armaghast to indicate the presence of a Christ-oriented culture on the planet, six hundred light-years from Old Earth and almost three thousand years before man left the surface of the homeworld.
In the time of the Pax, Armaghast is a quarantined world. Raul Endymion's final death sentence is carried out in a Schrodinger cat box in high orbit above Armaghast.