The Fesarius was a starship built and operated by the First Federation, encountered in the 23rd century. It was described as the flagship by its captain and only crewmember, Commander Balok.
The Fesarius was the ship that initiated first contact between the Federation and the First Federation in 2266, when it was encountered by the Federation starship USS Enterprise. 
Technical details and capabilitiesEdit
The vessel was very large, and spherical, with the surface made up of smaller, dome-like features, arranged in a distinct pattern based on triangles. The Fesarius was able to dispatch at least one small craft, described as a "pilot vessel", and could be controlled by just one crewman using a very simple looking interface.
The mass of the Fesarius was beyond the ability of 23rd century Starfleet instruments to register. Commander Spock estimated the vessel to "be a mile in diameter".
The vessel was able to scan the entirety of the USS Enterprise in a few seconds, in sufficient detail to then override and shut down some of its systems, a level of control that was described as "extremely sophisticated" and "brilliant".