After saying goodbye to the scorpion, William continued to lead the party, following the instructions of the computer map. They passed Checkpoint 39 and got 2% of their energy. After they passed the checkpoint, a dialog box popped up on their computer screen,

You have to go through the final checkpoint. The rest of the energy will be charged there. Perseverance is victory! Come on!

A faint smile appeared on William's lips. He clicked "OK" and moved on.

But the scene ahead was more and more frightening. The ghostly trees grew denser, twining their tentacle-like branches. The distant cries of crows would have petrified any man. The road winded up and down the hills under their feet. A few minutes later, a gloomy little house appeared in front of them. Ranran looked at it with trembling: although it was not high, with only two floors, yet the terrible atmosphere it emitted was not weaker than that Black Tower did. Its exterior walls were a complex of colors: black for nothingness, white for skeletons and red for blood. They piled up irregularly and seeped into each other. Seen from a distance, the house looked like a sleeping ghost that might wake up at any moment.

"Aargh..." Ranran's whole body was shaking violently. "Can we not go in..." Rebecca also asked William with some fear.

It seemed as if William's heart would fly out of his body the next second. But he shook his head reluctantly and said, "We have no choice but to go in."

He was right. A detour would have made them cover at least ten times further to reach the final checkpoint. It was apparent that no one wanted to do that.

"We're up to our old tricks, man. Go invisible." William turned to them all and said, "Everybody get close to me." The party quickly closed to William, took out the "completely disappear" clothes, and quickly put on. In the twinkling of an eye, they disappeared into the night.

Trembling, they walked to the door. Photon glimpsed some hints by the door,

The room is not tall, but it is large. You have to go out through the back door, and you need to find seven little keys to unlock it. But do watch out for the seven ghost skeleton robots in the house! They wander around different areas of the house. Every time you kill a robot, you'll find a key from its body. Kill them quickly with the most powerful weapons you have, and make sure none of them will find you! Otherwise, they will soon all gather and take your lives!

"Oh dear..." Natalie said to herself nervously. Since they were invisible, only she could hear herself now.

William went to the front and opened the door with a slow, almost nonexistent speed. The door creaked like the scream of a terrible ghost. Immediately, the party stopped moving and froze to the spot. Their blood froze, for fear of startling those robots. Time also seemed to be frozen, difficultly and slowly flowing. Fortunately, the compass did not emit a bright red light to warn them. Then they ventured into the house like snails. Aria came in last and closed the door more slowly than William, so there was a lot less creaking.

The room was eerily quiet, and the air was hardly moving. Everything here looked so horrible and gloomy. All the party could see were piles of bones or blood-dyed furniture. The sofa, which should have been soft and beautiful, was full of all sorts of skeletons, and the curtain had already taken "a blood bath". On the wall to the left, a clock was running feebly, while on the bookcase to the right, in addition to the mass of books, there was a frightening bone of hand, which was gripping fiercely in the direction where the party was now.

No one spoke, because everyone was too afraid to speak, even if the robots could not hear them.

William picked up the compass with a magnet at his collar. The compass had a very sensitive sensor. It could feel clearly that he was trembling with fear. So it consoled him by showing him the words,

Rest assured. If the robot comes, I'll warn you.

William, feeling better, touched the others and signaled them to move on. As they walked slowly, they kept looking around, on the lookout for the sudden appearance of robots.

Suddenly, Ranran heard a strange sound. It was like a bitter cry of falling into an abyss, or like a reluctant groan of a ghost. In short, it penetrated the ears of the party with a murderous atmosphere, straining their nerves.

The compass glowed bright red. "Get ready!" William signaled the others. He and Ranran pulled out laser guns. Photon and Rebecca loaded all their weapons. Natalie and Aria used portable laser guns. They were all looking nervously and intently in the direction of the sound.

Soon, two blood-red eyes appeared about ten meters in front of them. They glowed eerily and maliciously, as if they wanted to kill everyone. Little by little, it plodded toward them, though it did not know that anyone was there. When it got a little closer, Photon was able to see what it looked like. It was indeed a skeleton robot, made up of hundreds of bone-shaped shell parts and organ-like electronics inside the body. Its blood-red eyes glowered, its heavy legs moved slowly, and its terrible hands held up straight.

It came nearer and nearer, and William was ready to fire. "Fire!" He wrote on Rebecca. Within ten seconds, everyone had received the order. Then the plasma guns and cannons went off in unison, and a volley of red and green bullets flew out of the guns and into the robot. Definitely, it could not withstand the shower of bullets, and soon fell heavily to the ground. Its eyes lost their light.

"Yay!" Ranran was secretly happy. An opening appeared in the robot's belly. "That must be the key, huh?" William thought. The party nervously got close to the remain, finding that it was indeed a red key. Before they had time to think, the key stuck to the compass.

"Oh? What's the matter?" Ranran asked, puzzled.

"There are magnets in the compass. I think this key is magnetic, too." Said William, "Let's kill the other robots quickly and find the keys!"

"Yes, it's a terrible remain. Let's go!" Natalie trembled with nervousness.

However, William read some strange words on the compass,

"What does this mean?" He was talking to himself, yet apparently there was no clue to explain the message, so he had to put the question aside for the time being.

The party was once more in the terrible darkness. They got closer to each other because they didn't know when and where the robots would appear. The first robot actually gave them plenty of reaction time, but if they showed up just a few meters behind them, it would be pretty bad. They had to kill them before they could touch and notice them.

The compass glowed bright red again. The party stopped moving at once and looked around warily. Natalie saw the eyes and signaled the others to adjust their direction. But this time, there was something unusual about the robot's trajectory. Compared with the first robot, who moved in a straight line, it moved along a curve. It approached the party quickly, but slowed down and down until it almost stopped, then picked up speed again.

"Fire!" The robot fell down in a hail of bullets. With experience, they knew how to get the key. The orange key sucked on the compass. Now there were two keys hanging from the compass.

The writing on the compass became:

William was even more puzzled. What did these functions have to do with robots? "Wait a minute." He thought, "These functions might represent the trajectories of the robots."

He decided to look at the movements of the remaining robots to confirm the relationship. "If only these functions existed before robots appeared," He thought, "then we would be able to predict the best time to attack."

The party went on in the dark house. They walked from the living room to a bedroom. In the corner opposite them stood a large black cabinet. Its door suddenly opened and the robot jumped out. They saw it approaching the party faster and faster. However, it was unable to resist plasma bullets, which were traveling at close to the speed of light and carrying horribly high temperatures.

"Oh my God, I'm scared to death!" Ranran felt her heart beating violently.

"I prefer to walk in the living-room," Said Rebecca, still frightened.

"Me too..." Aria's wheels trembled.

From the remain of this robot, William got the yellow key. The compass showed these words,

This was somewhat convincing, because the third robot was indeed approaching the party faster and faster.

"It seems that my hypothesis is not without foundation." William said.

Before they knew it, they had reached everywhere on the first floor. Now a spiral staircase appeared in front of them. By the light of the compass, they could see that every step was full of blood. To make matters worse, they felt that the bloodstains were noticeably hotter than the rest of the area, as if they were still warm.

This made them all more nervous, for they suspected that a life had gone here not long before they came to the house. "The robots must be killed as soon as possible! They're too terrible!" Ranran was talking nervously to herself.

They climbed the stairs to get to the second floor. The back door was somewhere on the second floor.

William muttered, "Do watch out!"