(covers information from several alternate timelines)
"You fight for control of nations...we dominate entire worlds. We extend our will across time. If your race were to endure for a million years, you couldn't begin to approach what we've accomplished."
- - Vosk
The Na'kuhl were a species that evolved to gain a certain degree of control over time. During the 29th century, they were a faction in the Temporal Cold War – vehemently opposed to the Temporal Accord – and were led by Vosk, a dangerous fanatic who viewed time travel as his innate right despite the damage that consequently might be done to the timeline.
The Na'kuhl had pale complexions, bony-looking faces with bat-like features as well as bright red eyes, and many veins visible on their bald heads. (ENT: "Storm Front", "Storm Front, Part II") They also had yellowish-brown blood. (ENT: "Storm Front")
Weapons and other technology
The Na'kuhl were equipped with extremely powerful plasma rifles. Although these weapons required much power for portable usage, the attainment of such power was apparently very easy for the Na'kuhl in their native time period, where there were – according to an account of uncertain credibility that Vosk once made – energy cells as small as a coin. According to the same account, the Na'kuhl were additionally capable of unleashing a plague that targeted victims based on their genetic profile and could be dispersed throughout an entire country by introducing a few grams of a pathogen into that country's water supply. The Na'kuhl also used a style of hand-held communicator, in addition to the time-traveling technologies they developed at different points of their development. (ENT: "Storm Front")
The ancestors of the Na'kuhl under Vosk's leadership worshiped gods, and Vosk once referred to them as having been considerably imaginative but substantially less technologically developed. At another time, a Na'kuhl who Vosk later respectfully described as "our greatest scientist" stated, "Every moment we live, we are moving through time."
At one point in their history under Vosk's leadership, the Na'kuhl attempted to eradicate the Suliban. They traveled into the past to prevent the Suliban from becoming sentient. However, an opposing faction of temporal agents stopped the Na'kuhl and ensured that the Suliban attained sentience, despite the ongoing enmity between the Temporal Agents and the Suliban. (ENT: "Storm Front, Part II")
The Na'kuhl were very nearly defeated and captured by Daniels' faction, but managed to develop a type of stealth time travel before they could be vanquished. This innovative device had the drawback of allowing only one-way travel. Using the device, the Na'kuhl escaped to Earth in 1944 of an alternate timeline created by multiple temporal incursions from the different factions. In this reality, with the assassination of Vladimir Lenin by an unknown faction, Nazi Germany had been able to concentrate solely on the Western Front during World War II, allowing Adolf Hitler to quickly take over England and then much of the eastern coast of the United States, including New York City and Washington, DC. The Na'kuhl were allies to the Nazis and were granted ranks in the SS, complete with the "privilege" to wear SS uniforms as well as several Nazi medals.
The Na'kuhl further offered to provide advanced technological weaponry to the Nazis, but held back from doing so, not trusting the Human forces and being less interested in their war than they were. In exchange, the Nazis provided the Na'kuhl with materials to build a temporal conduit with which to return to the future, so that they could no longer be trapped in the past. In fact, the Na'kuhl were forced to create the conduit exclusively from materials acquired from that time period. The group was eventually successful and, just after Vosk was finally located by the Temporal Agents, the Na'kuhl used the conduit to journey to the 31st century, where they launched a decisive attack on Daniels' faction. This offensive not only enabled the Na'kuhl to defeat the opposing faction but also ignited the Temporal Cold War into an all-out conflict, resulting in the very same alternate timeline to which the Na'kuhl had escaped in the first place. (ENT: "Storm Front", "Storm Front, Part II")
In a last-ditch effort to restore the timeline and stop the Na'kuhl, Daniels transported Captain Jonathan Archer and the Enterprise NX-01 from 2154 to 1944. (ENT: "Storm Front", "Storm Front, Part II") Shortly after a wounded Archer was captured and hospitalized by the Nazis, the Na'kuhl had some involvement in overseeing his recovery. (ENT: "Zero Hour", "Storm Front") However, this degree of supervision was found, by Vosk, to be insufficient; after Archer escaped during an unsuccessful Nazi attempt to transfer him, another Na'kuhl named Ghrath was blamed by Vosk for neglecting to personally supervise Archer's transfer. Having discovered that Archer was from the future, the Na'kuhl time travelers at first came to the incorrect conclusion that he and the crew of Enterprise were Temporal Agents themselves. (ENT: "Storm Front")
The physical appearance of the Na'kuhl was generally met with shocked responses from citizens of the era and the unexpected presence of the aliens generated some stories about their existence, rumors that gangster and resistance fighter Sal learned of. The extravagance of these tales caused an associate of his, Alicia Travers, to initially disbelieve the reports. When Archer and the resistance cell traced the tales back to their source, an informant for Ghrath named Joe Prazki claimed that – even though he had only ever glimpsed Ghrath on one occasion – Ghrath's red eyes had been enough to leave Prazki with an unforgettable impression. For one resistance fighter called Carmine, the act of seeing Ghrath's Na'kuhl features caused him to almost utter an expletive in alarm and speculate that they might be simply those of a mask, a theory that was quickly denied by Sal. He referred to Ghrath as a "freak" and shot him to death. A later suggestion from Carmine was that the Na'kuhl had looked like a demon. Somewhat simplifying the group's struggle to identify Ghrath's appearance, Archer revealed that he had been an extraterrestrial. Sal's first instinct was to question whether the alien had originated from Mars, though Archer admitted to being unsure about what planet Ghrath had come from. (ENT: "Storm Front") From that point onward, Carmine considered the Na'kuhl to indeed be Martians. (ENT: "Storm Front, Part II")
The crew of Enterprise was ultimately successful in their mission, managing to destroy the conduit moments before Vosk could enter. This had the effect of eliminating the changes he had made, essentially resetting the timeline, and bringing the Temporal Cold War to an end. (ENT: "Storm Front", "Storm Front, Part II")
The name "Na'kuhl" is from script sources and was not mentioned on screen.
Although this species is first glimpsed at the end of "Zero Hour", the final draft of the script for that third season finale does not include them. Nevertheless, the appearance of an alien in Nazi uniform at the episode's conclusion – looking decidedly out of place in the World War II setting of a Nazi medical tent – was thought up by Rick Berman and Brannon Braga, who wrote the episode's script. (Star Trek Magazine issue 116, p. 17) The idea originally started with the members of the series' writing staff often joking amongst themselves that, following the end of the Xindi incident, the crew of Enterprise would finally return home to Earth only to find that the planet had been invaded by giant cockroaches. Certain only that they themselves were uninterested in such a twist ending being "a Xindi cliffhanger" (in Braga's words), the writing team began to seriously consider the possibility of an unexpected conclusion to the year. "We went through a lot of different scenarios about what they would find when they got back home," offered Braga. "I can't remember who said 'Nazis,' but we just somehow ended up with Nazis. Then that didn't even feel like enough, so we decided to make it alien Nazis. We decided to do something that would just be completely unexpected, yet give us something fun for [the] next year, to kick off the season with something really interesting. So we took a stab at it." (Star Trek Magazine issue 117, pp. 61-62)
Brannon Braga was aware that there had similarly been alien Nazis in both the original Star Trek series (Ekosians in "Patterns of Force") and Star Trek: Voyager (Hirogen in the two-parter "The Killing Game" and "The Killing Game, Part II"), also having been personally involved in the latter instance. (Star Trek Magazine issue 116, p. 62) According to Rick Berman, however, no-one on Star Trek: Enterprise's writing staff worried, upon introducing the Na'kuhl, that they were too similar to Star Trek's previous Nazi aliens. (Star Trek Magazine issue 116, p. 17)
Rick Berman was pleased with the audience reaction to the brief appearance of this species in "Zero Hour". "Well, I think we got a little bit of a shock and surprise, which is what we hoped for," he said. (Star Trek Magazine issue 117, p. 17)
During the lead-up to the debut airing of the fourth season two-parter "Storm Front" and "Storm Front, Part II", the alien Nazis were repeatedly rumored, on the Internet, to be Remans – who had previously been seen in Star Trek Nemesis – though these incorrect reports were quickly refuted by Rick Berman. (Star Trek Magazine Enterprise Supplement; Star Trek Magazine issue 117, p. 7) At the time, he stated, "I have read a lot of interpretations of what it might be but, in fact, we have not seen it before. I have seen people comment that it looked like a Reman, but this is a totally new alien." (Star Trek: Communicator issue 151, p. 13)
In a deleted scene from the episode "Storm Front", Sal privately hypothesizes to Archer that the alien seen by him at the end of "Zero Hour" may have actually been the result of one of numerous experiments that the Nazis were reported to be conducting on Humans, a possibility that Archer says he hadn't considered. (ENT Season 4 DVD)
Golden Brooks, the actress who played Alicia Travers and thereby appeared with the Na'kuhl in the "Storm Front" two-parter, was awed by the look of the species. "To see the prosthetic make-up on the alien Nazis; it was just out of this world, no pun intended," she enthused. "It's really, really amazing." (Star Trek Magazine issue 119, p. 98) On the other hand, production illustrator Doug Drexler commented that the two-parter "took some fan heat for 'space Nazis.'" 
- The Department of Temporal Investigations novel Watching the Clock has a faction of Na'kuhl from the 29th century stealing a Deltan time perceptor in 2372. It is unknown if it was the same faction that eventually fled to 1944.
- As revealed in the Star Trek Online mission "Doomsday Device", the Na'kuhl visited the 22nd century in the hopes of enlisting the aid of the Klingon Empire in the Temporal Cold War. They enticed the Klingons with information about the future, including the location of a disabled planet killer, but the Klingons refused them. A small faction within the Empire passed the knowledge down through the generations, culminating in the retrieval and activation of the planet killer by Ambassador B'vat's forces for use against the Federation.