Akuma Egg

The Akuma Egg, or Plant (生成工場 (プラント, Puranto)), is a strange device used by the Earl of Millennium to make Akuma.

Overview
The Akuma Egg is a giant blue, translucent, egg-shaped hunk of Dark Matter that the Earl uses to manufacture Akuma. It is mounted on a great, golden pedestal, hanging from a flower-shaped attachment on the top. Allen notes that the Egg seems to "breathe," and it also seems to have a noticeable pulse. The original Egg was located on the original Noah's Ark and, along with the rest of the Ark, was to be 'downloaded' into the new Ark being created by the Earl after the old Ark became faulty.

Noah's Ark arc
The Egg is first seen when General Cross Marian infiltrates the Ark and, using the combined powers of the Grave of Maria and his shape-changing abilities to disguise himself as one of its Skull guardians, defeats the Skulls. When Cross throws a Skull at it, seemingly to test how safe it would be to touch it, the Skull bursts into flames and, even in that state, taunts Cross, explaining that the Earl had cast powerful protective spells on the Egg and that only another sorcerer could break them. Seeing the truth in this after analyzing the Egg, Cross goes off to find help.

After saving and retrieving his apprentice Allen Walker and Lenalee Lee, Cross takes them both to the Egg's room, where it is being downloaded by the Earl, then casts a spell on the Egg to delay the download and sends Allen off to stop the download completely. Allen succeedes, and due to this interference, only twenty percent of the Egg was downloaded to the new Ark, making it useless. This, along with the Fourteenth's interference, makes the Earl visibly angry for the first time, an event that surprised the Noah Lulu Bell and made Lero hesitant and fearful.

Invasion of the Black Order arc
Once the Egg and the Ark are taken back to the European Branch Headquarters, it is revealed that four years prior Cross had been assigned the mission of destroying the Egg, four years also being the duration of time it's been since he had last been in contact with the Order. The Egg, itself, is turned over to the Science Division and moved to Laboratory 5 for study.

Without the Egg, the Earl was unable to continue making Akuma, so he sent Lulu Bell, the Noah relieved upon finding it safe. After killing most members of the Science Division, converting a few into new Skulls and capturing Allen, Lulu Bell opens an Ark Gate under the Egg and begins to retreat with it, only to have her efforts thwarted when Miranda Lotto uses her Time Record to recover the Egg. After all of the Akuma have been defeated, Komui gives the order to destroy the Egg, as it has already been taken and Miranda's power is just keeping it stuck in a time when it was still in the building.

Cross determines that the Egg is made of solid Dark Matter and determines that he, Winters Socalo and Klaud Nine should all attack it at once, but when Miranda releases her Time Record, Lulu Bell reappears, assuming a water form and capturing Miranda to protect the Egg. The Egg is damaged, and when Lulu Bell puts Miranda directly on the Egg to keep the Generals from attacking further, they unleash their most powerful attacks, anyway, damaging the Egg heavily.

As the Egg sinks into the Gate, Lulu Bell sitting on top of it, she notes that, even in its damaged state it can still be fixed, but just then Allen, who had jumped onto it to save Miranda, delivers a final, devastating blow to the Egg and destroys it completely. Covertly, Howard Link steps forward and retrieves one of the shards.

It is later revealed that the Earl is in the process of making a new Egg,

Artificial Exorcists arc
The shard of the Egg Link retrieved was used to make the Third Exorcists by implanting the shard in Second Exorcist Alma Karma and implanting the mutated cells that resulted from that experiment into five CROW: Madarao, Tewaku, Tokusa, Goushi and Kiredori.

Searching for Allen Walker Arc
An akuma mentions that akumas are springing at an incredible speed. This implies that the new egg is completed.