Yarnek, the silicon-based life form, is played by Janos Prohaska, who is the same guy who played the silicon-based Horta in "The Devil in the Dark". True!

Talk about typecasting.

Transcript

1
Captain's log, Stardate 5906.4. Surveying a lava planet.
Sulu: No life forms, Captain.
Kirk: Let's get out of here.
2
Lincoln: Oh, no you don't.
Kirk: Space Abraham Lincoln?
Lincoln: No, just Abraham Lincoln. The real one.
McCoy:
3
McCoy: No way that's the real Lincoln!
Spock: We have met the real Apollo, Zefram Cochrane, Jack the Ripper, and Leonardo da Vinci...
McCoy: Yes, but... This is... more ridiculous. Somehow.
4
Lincoln: Captain, Mr Spock, you must come down to the planet with me.
McCoy: It's lava!
Chekov: An Earth-like area just appeared below us.
McCoy: It's... an illusion!
5
Kirk: Our mission is to contact alien life, no matter how disingenuous they are or how obvious it is that they're tricking us into a deadly trap!
Kirk: C'mon, Spock.
6
{on the planet}
Spock: Surak. The greatest Vulcan, father of logic.
Surak: Live long and prosper, Spock.
Spock: This is not logical.
Surak: Nothing in this episode is.
7
Yarnek: Welcome to Excalbia. Behold! Four paragons of Evil: Genghis Khan, Colonel Green, Zora, and of course Kahless the Unforgettable.
Kirk: Who?
8
Yarnek: You will engage in an involuntary fight to the death. Good versus Evil! And your crew may watch.
Kirk: Just like the fight against the Gorn.
Yarnek: Damn those Metrons. Always stealing our ideas.
9
Surak: I will go talk to them. Surely four of the most evil and ruthless beings who have ever lived will listen to reason and make peace.
10
Caption: Surak inevitably gets captured.
Lincoln: You two create a diversion. I'll go rescue him.
11
Caption: Lincoln inevitably gets captured.
Kirk: We'll go rescue him.
12
Caption: Kirk and Spock win through superior experience with lame fight choreography!
Yarnek: I have learnt nothing about the natures of Good and Evil. You may go.