Ultimate Battle Swimsuit

Ultimate Battle Swimsuit is a game event in Mass for the Dead that was released on August 6, 2019 to August 20, 2019.

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

Quest
Momong 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 adrift in the sea, he believes the risk to exploring the island is worth it especially on gathering more information of the world. Given that the scroll was over hundreds of years old, it possibly 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 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 Momong has some reservations about the idea, the maids have already changed and are ready to deploy.

The Pleiades as 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 swarms 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 eaten both Narberal and Lupusregina compliment each other on their swimsuits. Lupusregina though 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 three big slimes to appear and the maids ready for a fight.

There fight is watched from afar by Shizu and reported 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 the see them. Yuri has Entoma use her summons to explore the seaside area in detail. After searching so far 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 decide 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 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 Rift, 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.

Aftermath
To be added...