I think this is a case where a comma isn't needed (grammatically speaking), but a speaker would introduce a slight pause when saying it out loud.
You're right that "and whose history defies any simple description." isn't a full sentence that can stand on its own.
But neither is "A land as old as time(,) and whose history defies any simple description.", as the main clause is still missing a verb. Regardless of the comma.
Either way, this is only half a sentence, meant to add further description about the land that the speaker is talking about. Grammatically not a full sentence, but allowed in terms of verbal speech.
Was this perchance spoken by a character in the narrative, as opposed to a description offered by the story's narrator?
It's often allowed to be grammatically imperfect in cases where you're quoting a character, as you might be quoting them ad verbatim. If they made a grammatical mistake, you'd have to add that mistake in the quote.
I met a caveman. He was holding a candle. I took the candle from his hand.
"You no take candle!", he shouted at me.
"You no take candle!" isn't grammatically sound, but if that's what the caveman said, it's how it will be quoted.