This was the first Klingon space vessel ever mentioned on Star Trek. While unseen during the original broadcast of "Errand of Mercy," it would later be established as a D7 class warship in the 2007 remastering of the episode.
The English script in the Klingon language edition of the non-canon comic "Against Their Nature" establishes this ship as the IKS Vortha, a ship of a class of Klingon scout vessels, similar in design to the D7 class but about one third the size.
Soon after, eight Klingon space vehicles, led by CommanderKor, engaged the USS Enterprise, driving it away from the system before they assumed orbit around the planet to deploy their invasion forces.
The two fleets were later prepared to engage each other outside of Organia, but would be stopped by the Organians, who made all instruments of violence on both of the star fleets radiate a temperature of 350 degrees, effectively ending the battle before it began. (TOS: "Errand of Mercy")
This fleet was unseen in the original broadcast, but would be established as being comprised of D7 class warships in the 2007 remastering of the episode.
In the original broadcast of this episode, the exact classification of this vessel was given as somewhat obscure. Kras identified his ship as "a small scout ship," nevertheless, the ship encountered by the Enterprise, which may or may not have been the same vessel, was recognized by Pavel Chekov as a "Klingon warship," and was referred to in the false distress calls sent by the Klingons as a "Klingon vessel."
Visually, it appeared as a flat shape with some sort of "handle" affixed to it. These inconsistencies were seemingly remedied in the 2007 remastered release of this episode, which visually established this ship as a D7 class warship.
The vessel was detected by the Enterprise arriving at Neural just as the Enterpriselanding party was beaming up to the Enterprise. Because the Enterprise was on the far side of the planet, the Klingon vessel did not spot the Federation ship, which was confirmed when the Klingon's routine message to their home base, which made no mention of Enterprise. To prevent detection by the ship, the Enterprise maintained radio silence, and after beaming a second landing party down, it left orbit to hide from the Klingon vessel until the Enterprise landing party could complete its mission. (TOS: "A Private Little War")
This vessel was merely mentioned in the original version of the episode and remained unseen until its appearance in 2008 as the recurring D7 CGI model, for what was its final Star Trek remastered television appearance.
After Kryton sabotaged the warp drive of the Enterprise, it was the job of the battle cruiser to strafe the Federationstarship in order to persuade it into engaging its warp drive and destroy the ship.
The ship was ultimately outmaneuvered by the Enterprise and limped back to Klingon territory. (TOS: "Elaan of Troyius")
In the remastered airing of this episode, the Romulan D-7s were donned with bird markings on their ventral hulls, much like that found on the Romulan Bird-of-Prey, which were not present in the original broadcast.
The novel The Romulan Way gives Talon as the name of the Romulan flagship in this episode. A sequel by the same author, Honor Blade, gives it as Memenda. These are presumably meant to be the same name, rendered in English and the Romulan language.
During a brief engagement, the Enterprise disabled the propulsion units of the lead cruiser, and the power supply of the second cruiser, leaving the crew unharmed. Following the engagement the Romulan cruisers to moved off, the latter, according to Spock needed several days to complete repairs. (TAS: "The Survivor")