Monsters (often called Infected) are the central antagonistic species in Kingdom. They are corpses that have been reanimated by the Resurrection plant or living beings that were infected with the plague. The infection evolved and soon became an epidemic that could make any living humans that had bitten transform into a mindless, cannibalistic creature.
The first monsters were created when Ashin, the last remaining Seongjeoyain used the plants discovered in Pyesa-gun to resurrect her own family and many villagers' corpse, chaining all of them in the barn.
After learning the truth of her village's massacre, Ashin killed some of the soldiers in the Chupajin to create monsters and destroy the outpost. After the incident, she vowed to destroy the Joseon Kingdom from within by selling the plants to any individual to spread chaos across Joseon, along with researching the plants and developing a stronger strain of the monsters.
During the Japanese invasions of Korean peninsula, Cho Hak-ju ordered his guards to kill all of Sumang Villagers (except Yeong-shin, because he already left the village) and created an army of monsters to fight the Japanese. After the war, Cho Hak-ju secretly researched the monsters to use them to protect Joseon from any threat.
After the King of Joseon died from an unknown illness, Cho Hak-ju, now seeking political power, forced physician Lee Seung-hui to use the fabled resurrection plant to bring the king back to life for a month. This was timed so that the King would remain "alive" until Hak-ju's daughter, the Queen Consort, was to give birth to a legitimate prince so their clan could become the new ruling power. Lee Seung-hui brings the king back to life, but he becomes a mindless beast and is kept chained in his bedroom while his death and current conditions are kept a secret from everyone.
In Hanyang, Dan-i and Physician Lee enter the royal palace under the cover of darkness to tend to the King. Physician Lee warns Dan-i to not look into the King's bedchamber. But when Dan-i kneels down to give the King a bowl of medicine, he is attacked and dragged into the bed while Lee looks on in horror.
At Jiyulheon, Lee's clinic just outside Dongnae, physician Seo-bi is contending with a severe food shortage as she treats her patients. In spite of government promises to send rice, the people are near starving. Seo-bi is relieved when Physician Lee returns, but he is obviously distressed and distracted. She is shocked when she learns that Dan-i died on the trip, and is disturbed upon seeing the savage bite marks on his corpse. However, Physician Lee refuses to tell her what happened and locks himself in his room.
Later, Seo-bi returns from gathering herbs to find that one of the patients, Yeong-sin, has cooked up a meat stew for the other patients, since they are starving. Yeong-sin tells her that he poached a deer. However, when Seo-bi finds a finger in the soup, she realizes it was made from Dan-i's body. Horrified, she confronts Yeong-sin, who tells her that people often resort to cannibalism in times of famine. As they argue, the patients suddenly begin to take ill, and within a short time everyone who drank the soup has died. However, they resurrect shortly afterwards as night falls, and attack and kill the clinic's staff.
Later on, Lee Chang and Mu-yeong enter the clinic and find it apparently deserted, but with signs of a gruesome battle. When a floorboard breaks beneath him, the prince is horrified to discover a tangle of bodies tied together beneath the clinic buildings. They alert the local guards, who recover the bodies and take them to the magistrate's office. Yeong-sin, meanwhile, discovers the bodies are missing when he returns to the clinic from gathering bamboo for defences. He rushes to the magistrate's office, where the coroner is examining the human bite marks on the bodies. He warns them that the people will resurrect, but the magistrate, Cho Beom-pal, orders his arrest. Yeong-sin attempts to burn the corpses, but is subdued by the guards and tied up.
At the magistrate's office, Cho attempts to obtain a confession from Yeong-sin, ignoring his protests about the bodies. Seo-bi enters with Mu-yeong, and attempts to convince the magistrate that he is telling the truth. When that fails, she tries to warn the assembled crowd to flee; however, they also dismiss her. The magistrate calls her and Yeong-sin lunatics, and orders that they be locked up. In that night, an acrobat notices one of the corpses at the magistrate's office moving, much to the surprise of the crowd and Mu-yeong. A man recognizes the woman as his wife, and runs to her side, where he is immediately attacked. The crowd scatters in a panic as the other bodies revive and begin to chase them.
At Jiyulheon, Cho Beom-il confronts Crown Prince Chang, having come to arrest him and bring him back to Hanyang. During their fight, his guards open a cabinet only to be attacked by an infected Physician Lee, who had been locked inside. Beom-il stabs him as he mauls a guard, but is shocked when he doesn't die. The infected guard then turns on him, and while Beom-il is distracted the Physician bites him too. The Crown Prince kills the physician, but Beom-il has already been infected and begins to turn. The second guard flees, locking the Prince in the shed with Beom-il; fortunately, the Prince manages to kill Beom-il by beheading him. The Prince escapes and runs towards Dongnae. He reaches the river, only to see the city burning on the opposite bank.
Knowing her father's secret works, Second Queen Cho assassinates Cho Hak-ju and captured Seo-bi, planning to use monsters to purge humanity and make Hanseong the last civilization on earth. However, when Lee Chang returns to overthrow the queen, she orders her servant to release captive monsters to kill everyone in the palace (and Hanseong) along with herself, but her ultimate plan fails when Chang lures every monster to the iced-over pond and, breaking the ice, drowns all the monsters.
Seven years later, former prince Chang, now a secret investigator along with his comrades, follow Ashin into one of her lairs in the forest, only to discover that they've been lured into a trap. At the same time, Ashin along with packaged monsters, are planning something vicious against Joseon.
They are mindless predacious creatures that will attack any humans in their sight, devouring their victims. Oddly, though animals and insects flee from them, the monsters only target humans. The monsters are generally fast-moving and agile, but will suddenly hibernate in hot environmental conditions, only to "reanimate" in cooler conditions. They have rabies-like symptoms such as agitation and fear of water. Beheading, burning, or drowning are the only ways to incapacitate or kill them.
There are three types of infected:
- Type A: The recently deceased who were resurrected by the resurrection plant. They always become blood-thirsty monsters. The victims of Type A however do not become monsters after death.
- Type B: Those who cannibalizes the body of a victim of Type A. Shortly after consuming the flesh of a Type A victim, the person will take ill and die, before reanimating as a monster. A victim of a Type B will reanimate as a monster after death, unlike the victim of a Type A.
- Type C: The bitten victims of Type B. After being bitten, they become very ill with decreasing body temperature until they die. The person will then reanimate as a monster and is capable of turning his victim into a monster as well. Type C and Type B is different only from how they acquire the illness; other than that, both types are indistinguishable from each other.
Other than humans, animals can become monsters by eating resurrection plants or eating the body of monsters. Infected animals are different from infected humans that they are more aggressive but still maintain intelligence. Animal monsters are almost indistinguishable from normal animals, the only difference between a living animal and an infected one being its pupils, with some animal monsters' eyes (especially herbivore) covered by a white or grey film of mucous either covering the entire eye or just the pupil.
In Lee Yeom's case, there are many parasites in his body but it still didn't turn him into a monster. It is currently unknown if he is immune or he is new type of infected.
- Tiger of Baekdu
- King of Joseon
- Lee Seung-hui
- Cho Beom-il
- Ahn Hyeon
- Cho Hak-ju (Cured)
- Second Queen Cho
- Lee Chang (Cured)
- Yeong-sin (Cured)
- Min Chi-rok (Cured)
- Cho Beom-pal (Cured)
- Second Young Prince (Possibly Immune)