Archernar IV is an artificial planetoid originally constructed as a type of explorer starship. In the 23rd century, it acted as a generational ship inhabited by sentient spider-like Archernarians and Crawlers, as well as a variety of other insectoid species.
Its diameter was described as comparable to Luna, somewhat near 2,160 miles. The vessel's hull blocked subspace communication and transporter beams. Tricorder scans were blocked by radiation within the superstructure.
Millions of years ago, an unspecified race of giants constructed the worldship as a vessel for interstellar and/or inter-dimensional exploration. For unknown reasons, the ship was abandoned in the Archernar system. Over time, insects living within the interior evolved to sentience, accelerated by long-term exposure to quadraphasic radiation.
In the 2260s, Archernarians launched a warp-driven starship from Archernar IV, which led to the instigation of Federation first contact procedures. Starfleet then constructed an observation post on Archernar IV. For six months, Captain Elizabeth Cassady commanded a survey team that discreetly studied the world and its inhabitants.
In 2264, the crew of the USS Enterprise visited Archernar IV to make contact with the Archernarians. Joining with Cassady, the shuttlecraft Niagara and Sutherland made a formal landing within the terraformed interior. While there, Spock realized the potential benefits and dangers of the vessel's engineering core. (TOS comic: "Mission's End, Issue 1")
Between 2264 and 2270, the Archernar IV contact station was installed in orbit. Its staff of 13 included Ambassador Cassady and Bill Hadley. A docking station with an airlock was also added to the hull of Archernar IV to simplify entry and exit.
In 2270, following a petition with the Federation Council, the Enterprise returned to Archernar IV for a ceremony to make Archernar IV a Federation Protectorate. As a landing party approached in the shuttlecraft Columbus, the high administrator's son Kel-M rebeled and engaged planetary defenses — an Omega-weapon that destroyed the contact station, killing 11 people. A small quantum singularity formed in the wake of the explosion, and Montgomery Scott was forced to eject the Enterprise's warp core, which drove the singularity out of the system. Kirk rammed the Columbus into the Archernarian control chamber, killing Kel-M and rescuing the high administrator. Kirk nominated Scott for a Pentares Ribbon of Commendation for his actions. (TOS comic: "Mission's End, Issue 2")
A rescheduled signing ceremony near Archernar IV's Heart of God was disrupted when an organized slave rebellion of Crawlers ran off with a section of the engineering core, which triggered an auto-destruct cycle. (TOS comic: "Mission's End, Issue 3")
Damage to the core began to produce a rift in pan-dimensional space. After the civil war was ended by the high administrator, and the pieces of the core restored, the engineering core powered up for a dimensional jump outside the prime universe. The remaining Starfleet officers left the station and fled the area alongside the Enterprise. A fleet of Orion interceptors remained, and were caught in the wake as Archernar IV vanished into another dimension. (TOS comic: "Mission's End, Issue 5")