After the protagonist was hailed a hero for driving away the Demon Emperor Jaldabaoth from E-Rantel, the homunculus hero brought a proposal to Momonga and the other Floor Guardians that it was in Nazarick's interest to care for the well-being of the fortress city. Having made tours to the new and old sections of E-Rantel, the protagonist made an observation that while humans and demi-humans coexisted in the new section, there were many people were still suffering from trauma of the Catastrophe in the old section. If left unattended, it could lead to a possible conflict between the two areas of E-Rantel. To avoid that scenario, it was decided that not just physical assistance to the city was needed but to provide mental relief. It also prompted an opportunity to seize the hearts of human inhabitants. Among the ideas of what method they should use, it was Momonga's suggestion of using idols that was put to play.
Mass for the Dead Arc
- Main article: Mass for the Dead Arc
Before implementing the idol event in E-Rantel, a trial run was agreed to be done within the Great Tomb of Nazarick and for the Homunculus Maids in consideration that they were similar to humans and to reward them for their efforts in maintaining Nazarick. Though it was suggested that the Pleiades would make good idols for this event, the roles were given to the Floor Guardians as the battle maids already had many missions to follow-up on. Two Floor Guardians were selected for the idol singers and made their surprise debut at the cantina on the 9th Floor where they gave a small show to the general maids.
Demiurge acting as the producer for the idol show and the protagonist as the manager together did some research on idol singing shows. In order to give the general maids a truly interactive experience Demiurge wanted to do a live idol show in a setting where the homunculi would feel a foreboding environment and be uplifted in spirit by the idols. The ruined Throne Room was selected to be perfect with the idols not just singing but also battling against the Chaos Beasts that emerged periodically. The plan was proposed to Momonga, though he had reservation of putting non-combatants in danger but after measures were taken to ensure the security and safety of the spectators, the Overlord consented to the plan. To test out the live idol show, a trial run was agreed before taking the show on the 10th Floor.
To start the first actual live idol performance took place on the 6th Floor in front of the Crack anomaly there. The event turned out to be a success by Momonga who viewed the scene via his undead summon. Plans to proceed to the show on the 10th Floor were approved and also the start of creating idol merchandise for the maids.
After commemorative goods were given out, such as autograph cards and hand-signed towels at festive locations like Spa Resort Nazarick, the second campaign for the idol plan went into effect. The next show took place in the ruined 10th Floor within the Throne Room before a crowd of general maids and their security guards. The crowd watched as the idols Acidity and Alkaline fought against the invaders from the Crack in awe at the fight and realizing the power the Floor Guardians face every day for them.
A few days after the start of the second drive of the idol plan, Cocytus finds a large line of maids outside the cantina being ushered in an organized fashion by Mare. There the maids are given more autographed goods and chance to meet the idols in person. Demi P seeing Cocytus and Mare are present convince the two to partake in a surprise live battle show with the idols much to the glee of the maids.
The idols take a break from the event and are visited by Albedo who came to give her thanks to the two of them for their hard work. Though she also adds the shame it is for them to be sacrificing their candidate places in the Queen War, as the literature in idol material clearly state no romances are allowed for them. In other words they were no longer qualified to be Momonga's queen due to the restriction. Shalltear hearing that piece of news was furious and led to her decision to disband the idol group at the end of the second campaign. The revelation was of course all part of the plan composed by Demi P so that the disbandment of LittleMass would spread among the maids and drive its popularity and fanfare through the roof. He hoped to produce an effect where the final show would produce the greatest turn out.
The final show takes place in the Amphitheater where a grand stage has been set based on specifications from idol literature. There the idols perform their last sow before their fans. The Corpse Guardian is brought out to be their newest opponent. Once LittleMass defeat it, Momonga personally comes out to show his gratitude and present the Staff of Ainz Ooal Gown to the audience and casts a spell to summon a Primal Fire Elemental as Acidity and Alkaline's last opponent for the finale.
Once the idol plan came to a conclusion, the general maids went back to their usual routine. Though they believed it was a pity the idol group disbanded, the show motivated them to work harder to repay the Floor Guardians and Momonga. The success of the plan convince many Floor Guardians that it was feasible to implement in E-Rantel. Momonga while admitting it was sound, advised that they should keep the idea on standby as E-Rantel contained religious forces like the Slane Theocracy who may take offense. Shalltear though was all for the idea to revive LittleMass on they day it was brought to E-Rantel, as she found a loophole in the no romance rule for idols, believing that scandals can happen but so long as she has talent she can still be with Momonga. Albedo irked by Shalltear's confidence tries to get Momonga to make her into an idol and is also followed by Mare leaving the Overlord at a loss on what to do.
LittleMass would be reformed again at the "Nazarick Swimsuit Athletic Meet" at the Open Bath Spa where Acidity and Alkaline provided a live mini-show for the interim between the second and final competition. Pandora's Actor was the special guest who battled against the two idols at the venue.
The idol unit was composed of two members of the Floor Guardians selected due to their appeal to the human demographic and cuteness. Despite that the two young women are capable of singing and battling simultaneously when doing their performance. Though their lyrical works were provided by music specialist such as Neuronist Painkill and Chacmool.
- The idol group may be a reference to Overlord: The Undead Oh! when characters from the manga believed that Mass for the Dead was an idol training game.
- The stage names of the idols relate to the pH value of water.
- Jūami, the artist for Overlord: The Undead Oh! expressed her wish two years ago of wishing "Obamasu was an idol training game".
- The name LittleMass is a reference to the idols who are the shortest statures among the Floor Guardians.