Enormous spherical flagship of the First Federation, commanded by Balok, which tested the Enterprise with a bluff and threat in The Corbomite Maneuver.
The Fesarius was the flagship of the First Federation, a gigantic glowing sphere roughly a mile across that dwarfed the Enterprise. It was preceded by a spinning cube that acted as a border marker, which Kirk destroyed after it bombarded his ship with radiation.
The ship's commander, Balok, declared that the Enterprise would be destroyed for trespassing and gave the crew ten minutes. Kirk answered with a bluff of his own, claiming his ship carried corbomite, a substance that would annihilate any attacker. Balok then sent out a small pilot vessel that seized the Enterprise in a tractor beam.
The Enterprise broke free and crippled the small craft, and Kirk beamed over with McCoy and Lieutenant Bailey to help. There they found that the menacing Balok on their viewscreen was only a puppet, and that the real Balok was a small, friendly being who had been testing their intentions all along. Bailey stayed behind with Balok as a cultural envoy.