David Richter

@Ιασων Actually it was a verb (used by no less than Milton) in the seventeenth century. The OED entry has nothing more recent than Tennyson.

@KK The OED doesn't have a usage of havoc as a transitive verb other than in pre-1800 poetry

@Xword Junkie The OED doesn't have a usage of havoc as a transitive verb later than pre-1900 poetry. It was used primarily in the seventeenth century, but is not a current English verb.

@Mean Old Lady The last time HAVOC was a verb, according to the OED, was the nineteenth century, and most of the examples are from the seventeenth.