It's not a sentence.
If you want to continue it with something, you could use commas to set off 'deep and dark' - e.g., "The cave, deep and dark, intrigued the children so much that they had to explore it." Or "The cave, deep and dark, was at the very end of the tunnel."
Or it could come at the end of the sentence: "I tiptoed into the cave, deep and dark." I don't think that sounds as good, because it's a slightly unusual word order, but it is possible, and it would need a comma after cave.