Ultimate Battle Swimsuit

Ultimate Battle Swimsuit (最終決戦兵器水着) is a game event in Mass for the Dead that was released on August 5, 2019 to August 20, 2019. The event was re-released on July 17, 2020 to August 11, 2020.

Background
An old scroll is discovered during an operation experiment with [Gate]. According to the appraisal, it is more than 100 years old. In order to obtain information on this world, Momonga orders the Pleiades to investigate, but it is an unknown seaside that extends beyond the [Gate].

Incident
Momonga, after listening to Albedo's report on available resources for the Great Tomb of Nazarick, learns that due to the Catastrophe the dungeon is not operating properly due to the damage. In order to ease the transportation of goods from their assets, Momonga decides to test transportation magic. Previously since coming to the New World [Gate]s can no longer operate and function properly due to the vicinity of the Crack in Nazarick. However, he decides to try again.

Thus he acquires the aid from Shalltear Bloodfallen who uses her magic on the 1st Floor to open a [Gate] to connect to Nazarick's newly acquired mine. To see if it's safe Momonga summons a Skeleton to go through the portal. The summon steps through the threshold, and Momonga uses his summons to see what's on the other side. Instead of being greeted with the sight of the underground caverns of the mines, he sees a sea and a sandy beach. Seeing that he has failed, Momonga laments on his failure, until Abledo points out that the Skeleton had returned with an old scroll in hand. Taking a look at the item the Overlord sees a drawing of the beach on the other side of the [Gate].

Unable to decipher the language, Momonga summons Pandora's Actor to appraise the item. In his initial examination of the artifact, the Area Guardian determines that he is unable to read the words on the scroll, due to the age and damage. Though from the material's condition and deterioration, he estimates the scroll to be at least over a hundred years old. With the [Gate] experiment a failure, Momonga decides to focus on this new terrain and gather information.

Momonga, believing that the scroll is evidence of some sort of civilization on the island, decides to launch an investigation. While it is possible the scroll may have been from the Crack in the sea, he believes the risk of exploring the island is worth it, especially considering what they could learn about the world. Given that the scroll was over one hundred years old, it could give Momonga clues on the connection this world has with YGGDRASIL. Since opening a [Gate] for an extended period of time was dangerous for Nazarick, a survey needed to be done in a short amount of time.

Thus the Pleiades were summoned for this mission and supported by Slimeko and the protagonist. Yuri Alpha voiced her concern that the mission would take place at the waterside, and possibly get the clothes given to them by the Supreme Beings wet. Since it was unsightly to have wet clothes, Yuri requested for her master to give them permission to conduct this mission undressed. Stunned by the request, Momonga rejects the request. Pandora's Actor however makes a suggestion to give the maids attribute equipment for this mission from the Treasury. Among the items are water resistant equipment that may prove useful for the mission. Momonga consents and Pandora's Actor returns and distribute equipment he claims were collected by the Supreme Beings: Swimsuits. Though Momonga has some reservations about the idea, the maids have already changed and are ready to deploy.

The Pleiades and their two allies cross onto the other side where they see the majestic view of a soft sandy shore. Yuri taking command, outlines the overall strategy. She separates the group into two teams that will survey the outer circumference of the island going clockwise and counterclockwise. The clockwise team will consist of Yuri Alpha, Solution Epsilon, CZ2I28 Delta, and Entoma Vasilissa Zeta. The counterclockwise team will consist of Narberal Gamma, Lupusregina Beta, and Slimeko. Additionally, Entoma summons a swarm of insects to scout the interior of the dense forest of the island. The two groups separate eager to complete their mission.

In Narberal's group, Slimeko takes her time enjoying the sea view. She soon notices that the ground in front of them on the beach is a little strange. The color has changed and is full of small holes. Thinking it might be a clue to potential artifacts, the slime digs but soon discovers it to be a swarm of crabs. Using [Electrosphere] Narberal roasts the crabs which are then eaten by Slimeko as a snack. While watching the slime eat, both Narberal and Lupusregina compliment each other on their swimsuits. However, Lupusregina wishes to actually test them in battle. Just as Slimeko tries to warn them of the potential dangers on slimes, she vanishes through a sand trap, having been caught by a slime. Lupusregina finding the scene hilarious decides to intervene. In the scuffle, Slimeko is freed from the sand but causes the maids to get splashed by water. To their surprise, their swimsuits are not wet thanks to the items' special ability. A trio of big slimes to appear and the maids ready for a fight.

Their fight is watched from afar by CZ, who reports to her team. Yuri leads the maids along the beach to a reef area. Finding the area to be infested with slimes, the group proceeds to destroy them when they see them. Yuri has Entoma use her summons to explore the seaside area in detail. After searching, all they find are small pieces of cloth or wood. There is a cove ahead with a cave-like side hole in the back and the group decides to check it. At the entrance, they peer into the cave and determine that it's deep and seems to be artificial. Thinking that it would be best to regroup with the other team, Yuri's group heads back to the rendezvous site after killing a slime that appears.

Back at the beach the two groups reunite and discuss their finds. Yuri notices that Slimeko looks full, and Lupusregina tells her its due to Slimeko being dragged in the water by wild slimes. Narberal reports that haven't found any evidence of a Crack, Chaos Beasts or any artifacts. Though they mostly encountered slimes and crabs attacking them. Yuri tells them of the cave her group discovered and wonders if they should investigate it. The protagonist reminds her that the forest's interior still needs to be checked. Using her insects Entoma calls her insects to check it out, and to the surprise of many they report that they found something. The forest now becomes a high priority and the survey team goes into its depths. At the center of the forest they find what appears to be a wooden board with screws. Disappointed by the find, it proves that their is a trace of human civilization on the island. Due to the tall grass it is impossible to search for any more clues. The maids suggest from burning the grass with fire or poison until the protagonist kneels down and eventually finds what appears to be a road hidden by the overgrowth. Following it leads to an old well. They approach it only for slimes to appear and attack. After dispatching the monsters, Slimeko volunteers to stick her body in to investigate. She confirms that it is deep enough for freshwater to be drawn up, confirming the maids' suspicions the island was in fact inhabited. They wonder where the inhabitants could be located, until Shizu mentions the cave. The Pleiades and their allies then travel back to the reef where Yuri and are greeted by a mob of slimes that are dispatched by Solution. Yuri has Entoma recall her insects in the area they left previously. Entoma does so and after conferring with her summons confirms that there is a deep path in the cave. However there seems to be something obstructing the passage in its depths, and due to the low intelligence of her insects is unable to ascertain what it is.

The group goes in and find it filled with bugs which are happily munched on by Entoma. They finally find that the obstacle some sort of elastic wall. Slimeko volunteers to check it out before anyone. She finds its scent strangely familiar and begin to take a bite out of it to the surprise of everyone. Soon the wall is revealed to be a Giant Crustacean. After it is killed by the Pleiades, moving it out of the way reveals that the cave goes deeper. The next part of the cave seems to be just a large burrow for small crabs and a dead end. At a loss the group despair, until Shizu begins using her Magic Gun at the wall. She points out that the wall is fake, noting that the numerous holes which are supposedly crab burrows are not at all inhabited by the crabs. Lupusregina pressing her ear to the wall, confirms that Shizu is right. To open it, Yuri uses [Impact Blow] and breaks it down, revealing another passage. The group continues on but notices that the atmosphere of the area has gotten colder. At first they believe it's because they are appearing to few clothes, but the group soon find the source...a horde of Skeletons in tattered pirate attire. Battling through the horde the group finally reach the final part of the cave, which turns out to be a hidden port. The port is in ruins, but they find even more undead an to their surprise a Death Knight. The maids take care of the monsters. Once they are finished off the undead they find there is a waterway leading to the sea. In addition they see the mast of what appears to be a ship in the water.

It becomes apparent the the undead were once pirates that manned this hideout, but were killed when their base was discovered as signs of a battle are found. In addition the mechanism that opened the wall they came in was destroyed thus trapping the pirates and their attackers in the cavern. As for the ship, the remains of it are found in the water. The group assume the pirates tried to escape by sea, but somehow it sunk. The group is concerned on the cause for the ship sinking, and believe an enemy may have caused it. However it's also the possible place where the pirates' treasure is located. Pressed for time, due to the [Gate] that is waiting for them to return Yuri goes into the water to check the ship. Yuri returns to the surface and confirms that she found the ship's hull. The next part of the problem is how to retrieve the artifacts and goods on the ship. With the limited time they have the best idea is to pull the ship up, but even the maids are at a loss of how to accomplish this. The protagonist however suggests that they block the waterway using the rocks from the ceiling and drain the water through the path they came through. Shizu and Yuri comply using their tools and skills. Once the water is drained, the wreck of the pirate ship is revealed. Noticing that the ship is broken in two, the group wonder why it's in that condition but are happy that can now accomplish their mission. Before Yuri can order her sisters to search for any relics, she is seized into the air by a seemingly invisible force.

An enemy present the other agents of Nazarick attempt to attack, but their attacks miss due to the opponent's invisibility. Yuri manages to free herself, briefly allowing a cluster of tentacles to appear on the cave wall. She lands an [Impact Blow], knocking whatever attacked her onto the ground. The dust soon settles and the attacker is shown to be a Giant Mollusk. Seeing it's powerful tentacles, the group realize this is what caused the destruction of the pirates ship. The maids unite in their attacks and eliminate the creature.

The groups turns to completing an investigation of the ship, with Slimeko gleefully finding a treasure chest. However it turns out to be open and empty. Though the chest is empty Entoma points out it could have been where the scroll found by Momonga's Skeleton may have originated. The group cheer Slimeko up stating that there are still plenty of artifacts that need to be transported.

Aftermath
Back at Nazarick, Momonga is pleased with the results from their mission. The cargo that they retrieved from the island contained artifacts like armor that confirm that the items are from two hundred years ago. The gold and silver will be used to for Nazarick's fund, and the wine found will be sold in the human markets for more money. From the logbook, Momonga learned that the island is isolated from the rest of the outside world. In addition the information he gleamed confirms that the technology found in YGGDRASIL was present two hundred years ago. He thanks the maids for their efforts. Yuri and her sister humbly accept the praise. They comment that it couldn't have been done without the swimsuits they were provided. They have nothing but praise on their utility and style. Albedo and Shalltear listening on the conversation eventually get into a fight on how they would look in swimsuits. Momonga explains that their swimsuits were not only used to enjoy a beach environment but also in the sea, piquing the maids' interest. The maids chatter wishing to go back and enjoy themselves on the island again in their swimsuits. Momonga watching the maids chatter about their time, brings memories of nostalgia he had with his guild mates.