The Killing Machine (1964) is the second of Jack Vance's "Demon Princes" novels, in which Kirth Gersen, having brought arch-villain Malagate the Woe to justice, sets his sights on Kokor Hekkus, another of the Demon Princes. The nickname does not refer to Hekkus's own predilection for homicide, but to his fondness for horrific and murderous devices, including a giant robotic executioner that first gained him his nickname.
To hone his skills, Gersen spends some time as a "weasel", a police spy in the lawless Beyond. He is sent on a highly sensitive mission in which Kokor Hekkus is reputedly involved. He is to intercepting a “Mr Hoskins”, with license to kill him if necessary. Mr Hoskins is killed, though not by Gersen, who however recovers two pieces of paper that were being exchanged. The one from Hoskins’s contact, Billy Windle, provides information on how to become a “hormagaunt”, a legendary undying monster that steals children and lives on the supposedly mythical planet Thamber; the other contains technical specifications that Gersen doesn't understand.
During debriefing, Gersen is glumly made aware that Billy Windle was Kokor Hekkus himself. He laments his lost chance, but shortly afterwards, he learns that Hekkus is masterminding a series of spectacular kidnappings, not his normal modus operandi. The victims are taken to Interchange, on a planet in the Beyond where exchanges between kidnappers and ransomers are facilitated. Gersen learns that the latest victims are the children of a local high-ranking Fellow of the powerful Institute, Duschane Audmar, who as a matter of Institute policy must remain aloof under all circumstances. Gersen convinces him to underwrite a fact-finding mission to Interchange to learn more of Hekkus’s activities, and incidentally to ransom the children.
While there, Gersen learns that Hekkus is accumulating funds in order to ransom a lovely young woman named Alusz Iphegenia Eperje-Tokay who claims to be from Thamber; she fled her homeworld when Hekkus became interested in her and settled on Interchange as her only possible refuge, since not even he would dare to interfere with that organization. As her own "kidnapper", she had set her ransom at a staggering, seemingly unattainable ten billion SVU, but Hekkus was undaunted, and set out to raise the immense sum by kidnapping the loved ones of the Oikumene’s hundred wealthiest citizens, ransoming them for a hundred million apiece.
Gersen also meets Myron Patch, an engineer from Krokinole. Patch had built Hekkus a unique “fort” made to look like a giant walking centipede, but when Hekkus was dissatisfied with the result, Patch refused to refund the money already paid, and was accordingly kidnapped and shipped to Interchange to recover the sum. Having enough money on hand, Gersen ransoms him as well as the Audmar children, and temporarily takes a controlling interest in his engineering company.
He determines to improve the fort to lure Hekkus within his reach, and has some ideas as to how to correct the faults Hekkus identified. Gersen demands and is paid more money by Hekkus’s agent Seuman Otwal for the alterations, but when it is time to deliver the completed fort, Gersen is taken captive and ordered to repay the money; when he cannot, he is dispatched to Interchange.
While there, Gersen sees an old newspaper article that identifies “Mr Hoskins” as a very senior bank official. He recalls the cryptic fragment of paper he recovered earlier and surmises that it describes marks that are used to authenticate banknotes. Acting on this information, he successfully forges enough money to free himself, and shortly after, Alusz Iphegenia. As she had in effect kidnapped herself, the money goes to her (less Interchange's fee) – and since she is uninterested, Gersen becomes fabulously wealthy. (Usefully, it is laundered money; Interchange and Hekkus are left with nothing, as Gersen's forgeries had been printed with disappearing ink.)
Gersen hopes that Alusz Iphegenia will be able to guide him to Thamber. She has no knowledge of astrogation, but is able to complete a nursery rhyme that allows Gersen to deduce the planet's location.
Thamber is home to a quasi-medieval culture, complete with barbarian tribes into whose hands Gersen and Alusz Iphegenia quickly fall. He has to fight the formidable leader of a war-band in order to save her from sexual slavery, and they accompany the warriors to Kokor Hekkus’s castle. There the barbarians easily defeat Hekkus’s foot soldiers, but then Patch's mechanical fort appears. It mimics one of their own greatest terrors, an animal called the dnazd, and they flee in panic before it. Gersen however had foreseen the possibility of facing Patch's creation and had installed an Achilles heel. He disables the war machine and takes its crew prisoner, in the process noting that one of the men aboard, Franz Paderbush, resembles Seuman Otwal and also Billy Windle, in height and build.
He takes the fort to the castle of Sion Trumble, at one time Alusz Iphegenia’s fiancé. Trumble offers the services of a friend who knows Kokor Hekkus, but the man denies that Paderbush is him. Gersen has his own suspicions. He allows his prisoner to escape, but then quickly forces his way into Trumble’s private quarters. There he finds Paderbush in the process of transforming himself into Trumble, for they are one and the same, and both are alter-egos of Kokor Hekkus. Hekkus himself has no face, having concealed his hideous un-face beneath a series of cunningly-made masks, and he had for uncounted years played numerous roles on Thamber in order to enact wars, conquests and atrocities for his own amusement. Gersen identifies himself, reminding Hekkus of the Mount Pleasant raid in which his home was destroyed and nearly all of his family killed, and after giving him a few seconds for the news to sink in, summarily executes him. He then returns to the Oikumene accompanied by Alusz Iphegenia, promising to send ships to bring Thamber back into contact with the rest of humanity.