Why is that so important did the producers of Star Trek had problems with Brill? Is Brill another in the line of people Gene Roddenberry manged to piss off?

No, just thinking of how weird it must have been, after all those years. And for such an unusual character.

It's like the possibly apocryphal story of a script being pitched to the P.I. show Barnaby Jones, starring Buddy Ebsen, for an episode where he goes looking for a missing multi-millionaire, Jed Clampett. (From "The Beverly Hillbillies." Also played by Buddy Ebsen, of course.)

Why not they brought back,Colicos,Ansara and Campbell

Yes but Darvin was more of a one-off, you'd think he might still be in a Federation prison or something. Or at least that they would have kept better track of him or something.

Kor (Colicos) was more of a "continuing character" even if he was only in one TOS episode. Same with Kang (Ansara), and Koloth (Campbell) although he was also Trelane of course. And Kang appeared in a Voyager episode, plus Ansara was in a second DS9 as a different character.

Well both Colicos and Ansera were both in only one TOS episode as well so I don't see the difference with Darvin

KlingonS were ongoing characters/threats/etc, but A Klingon modified to look human was a one-time thing?

Kang, Kor, Koloth, and their attachés etc, (Kang's) spouse(s), etc, were all KlingonS not just one character. But Darvin was the only Darvin.

That was prior to the introduction of the Ash /Vosq character in DISC.

Gotta ignore Discovery. Just too much hash.

Besides, wasn't that a "Klingon personality installed in a human?" Not a surgically altered Klingon.

@Knixon said:

Besides, wasn't that a "Klingon personality installed in a human?" Not a surgically altered Klingon.

It's the other way around and more complicated (as you can expect from "Star Trek: Discovery").

"Ash Tyler" was - like "Arne Darvin" - a Klingon (sleeper) agent, too. The Klingons had replaced the original humans, who were captured and, after extracting all the necessary information (including DNA and memories), killed.

"Arne Darvin" (Klingon agent name unknown) seems to have only been surgically altered, while "Ash Tyler" (Klingon agent Voq) underwent a more thorough transformation: ChoH'a'.

Not only was Voq physically (genetics-hybrid) altered to resemble the human Ash Tyler, but there was also a memory and personality transplant. Voq became an almost exact copy of the human Tyler, and only a very deep scan would've revealed something wrong with this "human". A further follow-up examination by an expert would've also been necessary to uncover the fact that he was really a Klingon masquerading as a human.

After ChoH'a' there were two personalities in the transformed Klingon: the original "Voq" and the transplanted "Ash Tyler". To be completely undetected, the "Voq" personality remained dormant (submerged), while "Ash Tyler" became - temporarily - the dominant personality. This meant that the Klingon Voq not only looked like the human Ash Tyler, but now Voq - unaware of his real, hidden identity/personality - also thought and believed that he was the human Tyler.

This would remain so, until the moment the original personality "Voq" resumed control.

However "Tyler" became attracted to Burnham. And, in short, the two personalities "Voq" and "Tyler" clashed and fought each other for control of the (transformed Klingon) body, and ultimately the "Tyler" personality won.

Thanks for the further explanation wonder on the differences between Ash/Voq and Darvin but it kinda proves the point that if the Klingons could do all the things they did to Ash turning Darvin into a human wasn't that difficult since his transformation was only plastic surgical.

Way over-complicated, of course. It would have been easier to just "re-program" the original human body, and then no kind of genetic manipulation to avoid scans would have been needed.

And if they had done the same thing with Darvin - and how did they get a "sleeper agent" into his position anyway, if it might have required YEARS or more of "climbing the ladder" etc - then McCoy couldn't have discovered his real "identity." It would have required something like a mind meld from Spock.

Even though it didn't fool those Tribbles Mudd had in DISC look how they react to Ash so basically the best Klingon detector is still a Tribble

Yes, and Tribbles wouldn't have detected a re-programmed human either.

@Knixon said:

And if they had done the same thing with Darvin - and how did they get a "sleeper agent" into his position anyway, if it might have required YEARS or more of "climbing the ladder" etc - then McCoy couldn't have discovered his real "identity." It would have required something like a mind meld from Spock.

That shouldn't be a problem. If it's a long-term mission , time is of less importance. Even now in a lot of TV shows or movies with (sleeper) agents it is revealed that they can start as young as children, and guided/fast tracked to fulfill a certain position in the enemy country.

And if you need an agent to be immediately available for a task, like someone to spy on the USS Discovery, then you capture and replace a crew member, in this case: "Ash Tyler".

A Klingonian Candidate