The Twelve Tasks of Asterix

After a group of legionaries is once again beaten up by the gauls, they imagine: "With such huge strength, they can't be human... they must be gods". Julius Caesar is informed, and laughs. He makes a decision with his council and goes to Armorica, to speak with Vitalstatistix. He gives the Gauls a series of 12 tasks, inspired by Hercules (but new ones, since the 12 Labours are outdated). Vitalstatistix assembles their best warriors, Asterix and Obelix, to do the job. The Roman Caius Tiddlus is sent along with them to guide them and check they complete each task.
The Twelve Tasks[edit]

Run faster than Asbestos, champion of the Olympic Games. Asterix, helped by the magic potion, follows Asbestos until the latter hits an apple tree at a moment of incaution, and loses the race.
Throw a javelin farther than Verses, the Persian. Verses' javelin hits North America (still only inhabited by Indians, including another Goscinny-Uderzo character, Oumpah-pah), but Obelix's javelin enters a stable orbit and pursues Verses around the world, into the Native American village.
Beat Cilindric, the German. Cilindric quickly beats Obelix with a "fighting technique he learnt in a distant land"; but Asterix asks for demonstrations that eventually leave Cilindric's own arms and legs tied in knots.
Cross a lake. The problem being that in the middle of the lake is the "Isle of Pleasure", inhabited by beautiful Sirens. Obelix comes to his senses after discovery that there are no wild boars on the island, and calls Asterix to follow him.
Survive the hypnotic gaze of Iris, the Egyptian. Iris tries to make Asterix act the phrase, "I am a wild boar"; but when Asterix constantly breaks his concentration by not taking things seriously, Iris hypnotizes himself.
Finish a meal by Calorofix, the Belgian. The chef is famous for cooking gigantic meals for the Titans - the task is to eat one of his massive three-course meals "down to the last crumb". Obelix devours a boar with fries, a flock of geese, several sheep, an omelette made with eight dozen eggs, a whole school of fish, an ox, a cow and veal, a huge mound of caviar (with a single piece of toast), a camel, and an elephant stuffed with olives. This exhausts the kitchen, but disappoints Obelix, who considers it "starters".
Survive the Cave of the Beast. In a distinctly abstract sequence of the film, the pair must enter a cave that no-one has ever emerged from alive. They encounter, among other sights, a skeletal hand that directs them, tennis played with a skull, bats, and a subway (on the Paris MÃ©tro station AlÃ©sia), before meeting the Beast (which is not shown on-screen). After they leave the cave, Tiddlus asks what the Beast was actually like - Obelix happily replies that it was "very tasty".
Find Permit A 38 in "The Place That Sends You Mad". A mind-numbing multi-storey building founded on bureaucracy and staffed by clinically unhelpful people who direct all their clients to other similarly unhelpful people elsewhere in the building. Asterix eventually beats them at their own game by asking for an imaginary permit, A 39 supposedly required by a new decree, "circular B 65", making the staff victims of their own unhelpfulness and sending the place into disarray. Eventually Asterix is given Permit A 38 just to make him leave and stop causing trouble, while the Prefect who gives them the form goes insane from the shock of his own unthinkable helpfulness.
Cross a ravine on an invisible tightrope, over a river full of crocodiles. Eventually the heroes fight the crocodiles and cross the river.
Climb a mountain and answer the Old Man's riddle. After a tough climb of the snowbound peak, the Old Man of the Mountain's challenge is to determine, with eyes blindfolded, which pile of laundry was washed with Olympus, "the detergent of the gods". Asterix performs this task easily in a parody of washing detergent advertisements. This last actually impress the Olympian gods themselves, until Venus suggests to grant godhood to the Gauls; but Jupiter refuses to do so.
Spend a night on the haunted plains. The plain is haunted by the ghosts of fallen Roman soldiers, is not an easy place to sleep. Obelix tries to fight them, but cannot harm the ghosts. Asterix is woken by the commotion and finally complains, until the ghosts depart.
Survive the Colosseum. When the pair wakes after a night on the plains, they find themselves in Rome with their fellow villagers, who have been brought to fight in the Colosseum. After the gladiators are beaten, the animals are sent in, and the Gauls turn the Colosseum into a modern-day circus.
After the Gauls succeed in every task, Caesar agrees that they are gods, gives them control of the Roman Empire, and retires to live with Cleopatra in a little house in the country. Caius Tiddlus takes his reward by retiring to the Isle of Pleasure. In the village's celebration, Obelix asks Asterix if the Gauls really did conquer Rome. Asterix tells him that everything that happened to them was a mere cartoon, in which everything is possible. Obelix takes advantage of this and teleports himself and his wild boar meat to the Isle of Pleasure, to enjoy himself.

Question: What is the plain haunted by?
Answer: The ghosts of fallen Roman soldiers