City on the Edge of Forever is regarded as one of the greatest episodes of the original Star Trek. The time travel story was written by Harlan Ellison, and is among the darker episodes of the original series, which was often lighter in tone and heavy on the silliness. The tragedy and uncertainty in “Forever” is on par with the paranoia and tension in “Balance of Terror”.
“Forever” also features one of the most under-appreciated an intriguing characters in Trek history: the Guardian of Forever. the Guardian is a sentient portal that can send the user anywhere in time and space, and is aware of the events of history as they unfold. His origin is mysterious. According to the dialogue in the episode, he had not interacted with anyone since at least before our sun came in to existence. He also implies that he exists outside of linear time.
I’ve been meaning to do a fanart of the guardian for a while now, and here it is;

Please click the image to see it in full-size. This was drawn on landscape paper with micron pens and copic multiliners. It was colored in photoshop.