Seven Lucky Gods

Seven Lucky Gods (七福神) or the Seven Pillar Gods (七柱の神) are a group of deities from another world and the creators of Shiramochi-ō.

Background
From Shiro's testimony, the Seven Lucky Gods existed in a world and were worshipped at a shrine. Originally they were just human beings, but in short, were praised as gods. In exchange for their lives and names, every time the New Year comes, the gods were suppose to continue to fulfill the wishes of others. Alas that role wasn't what the gods wanted or owed. Naturally the souls that worshipped them were not convinced. They rejected the desires of the gods. The gods were unable to escape their role to listen to the wishes of others who sacrificed themselves, and continue to fulfill them. When the time came, they wished—— they could fulfill their own wishes, so they created it separately. Their solution came in the form of Shiramochi-ō.

Mass for the Dead Arc
During a strange atmosphere of "New Year" that began to spread around the Great Tomb of Nazarick affecting the minds of the NPCs, Momonga sought to discover the source of the epidemic. The Overlord found his first lead in the form of the mysterious boy named Shiro who had no recollection of how he came to be in the Round Table Room. Momonga wary of the stranger allowed the boy to follow him not only in hopes of solving the mystery but to also keep an eye on his guest.

After soliciting the aid of the protagonist, Slimeko, Solution Epsilon, and Sebas Tian the group followed a trail of clues from several strange drop items that they collected from the various monsters and earning them from the mind controlled Floor Guardians. The items seemed to have a connection to Shiro who upon touching each one had a vision of their next destination. The protagonist eventually deduced that the items were associated with the Seven Lucky Gods and as they group reached the Bishamonten Shrine in Fake Nazarick it became clear that Shiro knew more about what was going on as he slowly regained his memories.

At the 10th Floor Shiro explained that the replica of Nazarick's Throne Room would serve as the stage of the Last Boss. Using the collected items he absorbed each of their powers, restoring him to his true identity as Shiramochi-ō The Ruler of New Year. Eventually Shiro revealed to the group that the entire situation was an event for the New Year and that the only way to restore everything to normal was to defeat him.

Momonga accepting this path, battled Shiramochi-ō, but as the duel progressed, he soon realized that Shiramochi-ō had no wish to actually fight. Truth be told it seemed that Shiramochi-ō wanted to intentionally lose in a way that Momonga would use an instant death spell on him. From what he could piece together, Momonga realized that Shiramochi-ō had encountered him and Nazarick before. Momonga postulated that the boy had either rewound time or shifted his existence from another world that was similar to the current. Shiramochi-ō impressed that Momonga pieced it together, stated that before when he fought Momonga his existence was unstable and thus he lacked a complete personality to fully grasp the situation. When he was born anew in this world, Shiramochi-ō's power was scattered around Nazarick. As he collected them he slowly remembered his grudge against his creators for making him into both a curse and a blessing every year. And the time he spent with Momonga and his friends, made him lack any motive to fight them and only wanted his existence to end so that Momonga's friends would be back to normal.

Momonga taking pity on the child who was callously used by his gods, decided to end the fight. Instead he offered Shiramochi-ō a last game of hanetsuki with Aura and Mare as he promised the former for a rematch. Shiramochi-ō stunned by the offer readily accepted to enjoy the last moments of his time alive among his newfound friends. Once that was done, he allowed Momonga to use [The Goal of All Life is Death] to truly kill him and thus preventing another loop of New Year.

A year had passed in the New World, which at the time a the celebration ceremony from Nazarick at New Year's attracted the attention of Shiro's creators who then sent another version of Shiramochi-ō to act as a raid boss against Nazarick. Shiro not willing to allow his friend be troubled by his copy, and also unwilling to give up his title as the Ruler of New Year, managed to hitch a ride through the summoning and return to Nazarick, this time though to aid Momonga and his friends.

Strength
According to the literature found in Ashurbanipal, the Seven Pillar Gods are described as beings who are said to bring good fortune. These gods are depicted in various scenes during the "New Year" as "lucky entities", and thus there are customs related to them. The gods possess seven items that are associated with them.

Ebisu

 * The God of Wealth
 * Item: Fishing Rod

Daikokuten

 * The God of Commerce
 * Item: Mallet

Bishamonten

 * The God of War
 * Item: Sansageki

Benzaiten

 * The Goddess of Music
 * Item: Biwa

Jurōjin

 * The God of Longevity
 * Item: Cane

Hotei

 * The God of Fortune
 * Item: Treasure Bag

Fukurokuju

 * The God of Wisdom
 * Item: Scroll

Trivia

 * The Seven Lucky Gods are a reference to Japanese deities.
 * Of all the Seven Lucky Gods, only Bishamonten is actually named in Mass for the Dead.