Vegan Infiltrators
This comes from a throw away line in TOS "Tomorrow is Yesterday" where Scotty mentions that the warp drives are fixed, but there's no place to go with them, since the whole are is dominated by the Vegan Tyranny in the 1960s
But by the 2060s, the Vegans were gone without a peep.
In Jay-Trek this works something like this -
The Vegans were a race fond of conspiracy and subtle maneuver. They never were actually an empire in the classic sense. But exploring records of Federation worlds it becomes clear that many Founing worlds were visited by the Vegans and ... interfered with.
The Vegans most of all wanted to prevent the rise of a competitor culture and so their goal was to retard progress by likely space going races.
On Earth and on mny other likely worlds, they sent Infiltrators. These were genetically designed beings who could change shape on a limited basis, and masquerade as natives among the race they were sent to disrupt.
Working from inside, these infiltrators created or stole likely identities and then sought positions of power from which to monkey wrench and forestall technolgical progress.
They were successful in many ways and in many specific instances.
They began landing on Earth in the 1940s and were discovered as aliens many times, most notably by Architect David Vincent who helped arrange a resistance movement to them in 1968. (The Invaders)
The Infiltrators were designed to avoid outright confrontation but to work in the Vegan way, using cheap backs shots and manipulation instead.
(See also the 1996 Charlie Sheen Movie, The Arrival)
Infiltrators were equipped with self destruct devices. When all neural activity ceased, the Infiltrator's body would break down into a fine black ash.
Infiltrators often undertook deep cover missions of such intensity that they were brainwashed into truly believing themselves to be earth people.
Some Infiltrators went mad, and changed their shapes into hideous things and went on rampages against their enemies, the humans. Often this ended in death for the humans, and the end of the infiltrator in a hail of bullets.
But, by 1991, the Infiltrators currenty on Earth were cut off for reasons unknown. They continued their mission, but attempts to find out what happened to their masters proved futile.
They went a little nuts, some striking out for themselves, some luring hapless passers by to their deaths and hijacking their ships. Some Infiltrators went native and decided they liked being human, a weak point of their creation was that they weren't really raised as Vegans and so after a period of lying like rugs they'd occasionally stop and ask "Just what is the truth?" and discover that they had no real idea.
By the 2030s the vegan infiltrators had lost all cohesion. The last one known to crypto-historians in Jay-Trek died in the 2080s, working as an advisor for the CIA.
Accounts of the terrible Vegan Empire available in some histories are regarded as misinformation designed to make Humanity more fearful and less eager to get out into deep space.
Vulcan and Andor have similar accounts of mysterious infiltrators who sought to keep space travel from advancing.
The true originators of the Vegans and their homeworld have never been identified
Examples would be - The aliens in The Invaders, The Aliens in "The Arrival" and The Smoking Man from X-Files.
Disclaimer: Paramount owns all things Trek. I claim original characters and situations in this material for me.