The pilot vessel used by Balok was a small ship capable of enormous power. It controlled the flagship Fesarius and was equipped with a tractor beam. Other than Balok himself, no crew was necessary.
On stardate 1514.1, the USS Enterprise was forcefully towed by the vessel on a heading to Carpi and could only free itself under great risks. (TOS: "The Corbomite Maneuver")
The script of "The Corbomite Maneuver" made it clear that Balok's pilot vessel was a section of the
Fesarius which detached from that mother ship.
The set for the interior of this craft was built on Desilu Stage 9, and was a redress of the briefing room set. (
Information from "The Corbomite Maneuver" call sheets, These Are the Voyages: TOS Season One, p. 126) Clint Howard – who, in his childhood, played Balok – approved of the set. He remarked, "
A great thing about the set is it was built to my size... and everybody else had to hunch down, which I thought was pretty cool." ("Inside the Roddenberry Vault, Part I", Star Trek: The Original Series - The Roddenberry Vault special features) Several walls from this small set were subsequently reused, to represent the M-11 Starbase Club in "Court Martial". (The Star Trek Compendium, 4th ed., p. 46)