They walked down a steep flight of stairs as Lucius was guiding Lord Voldemort to the cellar. At the bottom was a heavy door. Lucius unlocked it with a tap of his wand, and the creaking sound of the door opening was heard. It revealed a humid and musty room, left in total darkness.
The cellar was mainly used by the Malfoys as a room to store the family's collection of Dark artefacts. It was hidden below ground level in this room so as to prevent it from being seized during a Ministry raid. Now, as raids were a thing of the past and Lucius Malfoy had gained the trust of the Ministry, he emptied the room and moved the dark artefacts back into the upper room.
Entering the dark room, Lord Voldemort looked around before applying two charms.
Anti-Apparition Charm and Anti-Disapparition Jinx.
While one was a charm that prevented people from Apparating in an area, the other prevented wizards from Disapparating, as it aided in trapping a victim within an area.
During Lord Voldemort's stay in the manor, he wanted to change the function of this room to that of a holding cell for those he desired to capture. With those two charms, any wizard will find themselves trapped inside.
After personally doing these two spells, Voldemort took out a small box from his robes. While holding the box, it looked very tiny in Lord Voldemort's palm. Then the Dark Lord moved his wand again, and the Engorgement Charm (Engorgio) was applied.
The box in Voldemort's hand grew until it was large enough that it could fit both feet of an adult. In fact, this was the original size of the box, as Voldemort had previously used the Shrinking Charm (Reducio) to make the box so tiny.
By the side, Lucius was observing carefully but could not understand what the dark Lord was doing. He first thought that the box held a dark artefact, but his master did not take anything out of the box. Then, to his surprise, Lucius saw Voldemort step inside the box and fall directly inside. For a moment, Lucius stood on the spot, staring at the box.
Lucius was still trying to comprehend what was happening when he saw the dark Lord emerge from the box, and floating in mid-air next to him was a body. Lucius circled around a little until finally, he saw the identity of the person.
Harry Potter.
While the whole Ministry of Magic and even the Order of Phoenix were searching for the boy who lived, here he was floating unconscious, now inside his own mansion.
After the event in the graveyard, Voldemort decided to make his own temporary place to live and kept the boy a prisoner. Voldemort got the idea from the new memories where he saw how the wizard Newt Scamander made use of his briefcase to store magical creatures.
Voldemort, his red eyes glinting with satisfaction, placed the unconscious Harry Potter gently on the cold stone floor of the cellar.
Lucius, still bewildered by the display he had just witnessed, watched as the Dark Lord, with a serpent-like grace, approached him and inquired, "Do you have anything that once belonged to the previous house-elf, Dobby?"
Lucius, hesitating for a moment, considered the request. The memories of the peculiar house elf who had served the Malfoy family in the past flashed through his mind. But Lucius has never kept tabs on such a creature. He could not just say no.
Instead, Lucius turned and ascended the steep stairs, leaving Voldemort alone with the unconscious Harry. In the upper levels of the mansion, Lucius summoned the new house-elf that had replaced Dobby. The small creature appeared with a crack, its large, bat-like ears twitching nervously. Lucius, his voice commanding, instructed the house-elf to bring anything that had belonged to Dobby.
The new house elf, eager to please its master, scurried off and returned moments later, clutching a worn and dirty piece of cloth. Lucius examined it with mild disdain, recognizing it as a simple piece of fabric Dobby had left behind. The house-elf presented the item to Lucius, who took it with a nod of approval.
Returning to the cellar, Lucius handed the dirty cloth to Voldemort, "My Lord, I have only found this. I hope it is ok".
"This will suffice." Voldemort received it with a calculated smile.
Then the dark lord began to place a powerful evil curse on the piece of cloth. What Voldemort needed was anything that belonged to the low-life creature. The reason behind such an action was that house elves used a different magic system compared to wizards, and thus, the Anti-Apparition Charm and Anti-Disapparition that Voldemort had put on this cellar would not affect them.
Voldemort saw such a thing that happened in the future, where the house elf Dobby came to rescue Harry Potter in the very same cellar.
"My Lord, may I inquire why you need such a low-life creature's belongings?" asked Lucius, not understanding the reason behind his Master's behavior.
Voldemort smirked, "This is in case someone tries to foil my plans".
"Talking about plans, come with me, Lucius. I have a task for you. Your redemption begins now. But before that, let's gather all the Death Eaters together." As the dark lord Voldemort began moving, he took what looked like a newspaper from the magical box and threw it near the body of Harry Potter.
Walking closely behind, Lucius was wondering what sort of task his Lord had in reserve for him.
"I hope that I must not explicitly say to you, Lucius, that this area is off limits to others and no one should know what's inside without my permission."