You never include new information in your conclusion. Everything that appears in your conclusion must have been mentioned in your introduction or body paragraphs.
Remember an essay is not supposed to contain suspense, the marker should not get all the way to your conclusion and then be surprised by a new piece of evidence. If they wanted suspense or surprise, then the marker would have picked up the latest thriller and not your essay.
What if I have just remembered a really key piece of evidence that I absolutely must put in my essay?
Go back to your body paragraphs and fit it in. Then if it is really that important it should also appear in your introduction. Makes sure that it fits; don’t just tack into onto the end of a paragraph. But most importantly, don’t add it only to your conclusion!
In conclusion, never ever include new information in the conclusion of your essays.