The havoc they wreak or wreaked or wrought? For some reason the word havoc loves to snuggle up to the words wreak and wrought, and since Iceland’s now-famous volcano has been causing havoc, I’ve seen wreak and wrought far more than is normal. Both are correct.
So what’s the difference? Wrought has a nice gothic touch to it, I think, which seems to suit dragon-breathing works of nature. Wreak/wreaked is more common. The sense of the two words as verbs is now generally considered the same, with wrought sometimes thought of as the past tense of wreak. The first definition for both is generally “work”.
They didn’t start out together, with wrought once having a clearer sense of worked, while wreak is often used to mean cause, to bring about.
Examples of use:
Volcano wreaking havoc on science meetings, Science Magazine
The volcanic ash cloud that wrought havoc . . ., Metro.co.uk