A Knight's Ark is an essential piece of equipment that a Pactmaker requires in order to use a Knight. Arks take the form of a piece of equipment connected with the respective Knight.
A Knight's Ark becomes bonded to the soul of the first person who claims it and is deemed worthy; thus, the person becomes a Pactmaker. Once bound to a person, the Knight's Ark does not allow itself to be touched by another and reacts with violent flares of power when this rule is violated. The Pactmaker is bound until death. If a Pactmaker dies, a new soul may bind itself to the ark.
The power of the Arks and Knights was sealed away after the Dogma Wars years before the storyline is introduced; however, the Magi are determined to break the bonds that bound this ancient power and use it to control the world in the hopes of bringing about the rebirth an empire long since past.