The Na'kuhl were humanoids ressembling bat-like features with parchment-yellow skin, bony-looking faces with bright red eyes and bald heads with many visible veins. They are also notable for being one of a handful of species with iridium-based blood. Because of that, they are extremely light sensitive. (ENT episode: "Storm Front"; ST video game: Star Trek Online)
The Na'kuhl people were extremely isolationist, and had rejected almost all diplomatic contacts throughout their history, although they were rumored to have trade agreements with the Breen Confederacy by the 25th century. (ST video game: Star Trek Online)
In 2410, a Tholian fleet which had stolen the Tox Uthat from a time traveling Kal Dano headed for the Na'kuhl system in order to use the quantum phase inhibitor to destroy the Na'kuhl sun. Despiter the efforts of Kal Dano another Alpha Quadrant Alliance ship to save the star, the Tholians were succesful. As a result, the Na'kuhl isolated themselves even more from the outside world in order to rebuild their civilization, after rejecting assistance offered by the Alpha Quadrant Alliance to deal with the disaster. (STO mission: "Stormbound")
From that time forward, the Na'kuhl made their primary goal to ensure that time travel would never threaten their civilization again.
By the 29th century, the Na'kuhl had acquired the means for time travel, and were a faction in the Temporal Cold War, being against the Temporal Accord. (ENT episode: "Storm Front") A Na'kuhl faction utilized time travel to raid a Carreon ship in 2372 and steal a Deltan time perceptor. (DTI novel: Watching the Clock)
In the alternate timeline that was greatly affected by the Cold War, they became enemies of the Certoss, with the Na'kuhl tampering Earth's history and making Humans a threat to the Certoss, which led to the destruction of Certoss Ajahlan in that timeline's future. This led to some Certoss from that future to travel through time to stop the Na'kuhl and wipe out humanity, even when the war was effectively over in the primary universe and the timeline was restored. (TOS novel: From History's Shadow)
At some point, a Na'kuhl faction contacted the Klingons, and gave some of them knowledge of the future in an attempt to manipulate them into doing their bidding. However, the Klingons rejected this and rebelled against the Na'kuhl. Among the knowledge they had gained was the location of a Doomsday machine, which B'Vat later found in the T'Ong Nebula. (STO mission: "The Doomsday Device")