Ambimorphs were a technologically advanced race of telepathic shapeshifters from a distant corner of the galaxy. By nature the Ambimorphs were peaceful and generally non-aggressive, and found aggression distasteful.
The species had the ability to exactly replicate any object or any individual, right down to their molecular structure. Using telepathy Ambimorphs were able to perfect their disguises of individuals to the point where it would be impossible to tell them apart from the individual they were disguised as.
At some point prior to the 24th century the Ambimorphs constructed a generational starship the size of a small planet. This ship was sent to the Alpha and Beta quadrants of the galaxy. Upon arrival they found the local powers – the Federation, the Klingon Empire, the Romulan Empire, and others at war with each other. They placed their ship in orbit around a star and began investigations of the area.
Meanwhile, a Federation starship named USS Independence had located what appeared to be a small planet that wasn't on any of the charts in the Neutral Zone. Risking an incident with the Romulans the Independence traveled to this apparent planet and discovered it was hollow. They sent teams down to investigate this world. Meanwhile the Ambimorphs disguised themselves as inanimate objects and observed the Federation team. On board the Independence the Captain prepared a dispatch to Starfleet, figuring that they would be engaged in a long term study of their find.
Once satisfied that the Independence crew was not a threat the Ambimorphs revealed themselves to the crew. The Ambimorphs were warm and welcoming to the Independence crew. What the Ambimorphs hadn't realized was that they were carrying a deadly virus within them. By then a good portion of the Independence crew was infected, and a number of them had died by the time the Ambimorphs had destroyed the virus within themselves and a treatment was devised for those infected. The crew realized that they could never leave.
The crew of the Independence beamed down to the Ambimorph ship. A small shuttle was sent with four Ambimorphs disguised as single crew members who did not have families in the Federation. This shuttle was later discovered with the passengers apparently deceased. The bodies were returned to the Federation, whereupon the Ambimorphs changed disguises and infiltrated the Federation in order to study it closer. The Ambimorphs also got their ship designated as Hermeticus II, a quarantined world off limits to Federation ships. They also buried all information of their ship by highly classifying all records of the ship's existence.
Over the next 30 years the Independence crew made new lives for themselves on the surface of the Ambimorph vessel. The survivors made families and had children and eventually grandchildren.
While posing as a Vulcan ambassador, Subcommander Selok discovered the records of Hermeticus II. The IRW Syrinx was sent to investigate, with a captured Enterprise in tow. The Romulans were captured by the Ambimorphs and transported down to Hermeticus II.
Fearing that others would follow, the Ambimorphs decided it was time to return home. They towed the Independence and the Syrinx to another area near the Neutral Zone and destroyed both ships. A small team of Ambimorphs were left behind, disguised as Romulan survivors, with the intent of working to transform Romulan society into a more peaceful member of the galactic community. (TNG novel: The Romulan Prize)