Tarquin was a tyrannical king in 6th-century BCE Rome. Shakespeare is alluding to a well known account describing how Tarquin stole through the palace in the middle of the night to forcefully “ravish” the noblewoman, Lucrece, an atrocity that triggered his overthrow.

Shakespeare personifies murder as Tarquin sneaking toward his victim. On guard is a wolf, whose howl is his watchword — the call he makes while performing his rounds.

(Lucretia and Tarquin, Titian, c. 1571)