The Coffin of Eternal Darkness「常闇の棺 Tokoyami no Hitsugi」 is a powerful ritual relic that was used to seal away the Demon Clan for the past three thousand years after their defeat at the hands of the allied clans.
The relic takes the appearance of a circular relief that contains four fragments each representing one of the four clans that united against the demon clan with the Dragon Handle placed in the center.
Three thousand years ago, when the Four Clans emerged victorious over the Demon Clan, the giant craftsman Dubs  created the Coffin of Eternal Darkness to seal them away and cut off their realm from the physical world, bringing the land of Britannia to an era of peace ruled by humans whilst the Goddesses, Giants and Fairies withdrew back to their respective realms. The Coffin then broke off into fragments in hopes that the Demon Clan remained sealed. However, a few Demons, like the Red Demons, managed to escape the seal and went off separate ways either wreaking havoc in Britannia or plotting the revival of their Clan.
After being freed from his seal at Danafor, the demon Fraudrin assembled the pieces of the Coffin with the help of wizards who worshiped the demons. However, the seal of the Coffin would only be undone with the blood of a Goddess. Since the Goddess Clan had disappeared, Fraudrin tried to use the blood of the citizens of Danafor to perform the ritual to open the Coffin, but failed. However, finding Liz, Fraudrin recognizes her as the reincarnation of the Goddess Elizabeth and that with her blood he could complete the ritual. However, Meliodas appears and destroys Danafor completely by plunging into his rage for Liz's death.
After Danafor's destruction, Meliodas found the Dragon Handle, the key to the Coffin, and was tasked to guard it at all costs with his life until he later lost it to Helbram in their chaotic battle in Vaizel. Upon the completion of the gathering of fragments of the Coffin of Eternal Darkness, with the addition of the newly-claimed Dragon Handle, the Coffin was complete although it only managed to barely open the rift of the Demon Realm for the demonic entities to get through, to which Hendrickson concludes that Elizabeth Liones was needed for the completion of the ritual. Despite his defeat at the hands of Meliodas, Hendrickson fled to Zhuhur Valley with the Coffin of Eternal Darkness. Nearly dying, Fraudrin appears possessing Dreyfus and heals him, which allows the former Great Holy Knight to finally complete the ritual with the bandage cloth containing Elizabeth's blood. Although it was incomplete, the ritual succeeded and nine demons were released from the Coffin, but the majority of their powers were severely drained and so they head to Edinburgh Hill to recuperate. The shockwave also purified the demon blood within Hendrickson (a booby trap intending to kill whichever demon opened it), returning him to human status.
It is later revealed by the Lady of the Lake that Merlin was the one who gave Dubs the idea for creating the relic in order to ultimately seal away the Demon King and the Supreme Deity in her plan to resucitate Chaos, but she later realized that in order to break the seal on Chaos, one of the two gods had to be destroyed to distort the balance between light and darkness.
The Coffin of Eternal Darkness is a very powerful seal formed by the magics of the four clans that has kept the Demon Clan from entering the physical realm for three thousand years. The side effect is that the majority of the demons' powers was severely drained to a Nil that they needed recuperation. Despite its sealing power, a number of demons managed to escape from the seal. It has been booby trapped with magic that kills anyone attempting to tamper with the coffin. Fraudrin stated that had he opened it he probably would have been purified, which is why he used Hendrickson to open it for him. Hendrickson's demon blood was purified by the trap after he released the Ten Commandments, restoring him to a human.
In order to break the seal, the Coffin requires all its four fragments gathered with the Dragon Handle and the blood of the Apostle of the Goddess as the key to the release of the Demon Clan with a ritual that requires an incantation of an ancient language of Britannia.