Supreme Hanami (至高の花見) was a holiday event hosted by the Great Tomb of Nazarick in Mass for the Dead. It was released on April 4, 2019 to April 15, 2019. The event was re-released on April 17, 2020 to April 27, 2020.
Momonga feeling stressed with all the duties as Supreme One for the Great Tomb of Nazarick, reminiscences of the time when he and Herohero attended a Cherry Blossom Festival in YGGDRASIL. Even though the flowers and food at the festival were virtual it was a good escape from the cruel reality of work. He accidentally let out that he wishes to see those flowers again in front of Albedo. He tried to pass it off as letting his thoughts run free, though Albedo believes that it was a comment concerning her lose control of her emotions.
Albedo eventually met with the other female Floor Guardians and asks Shalltear Bloodfallen and Aura Bella Fiora about Hanami. They were unfamiliar with the term and Albedo explained that their master mentioned about seeing flowers and were confused as to why. Demiurge comes and asks what are they talking about but he was unable to give any insight. Thinking it to be a special genus of flowers they asked the protagonist in Ashurbanipal for more information. There the protagonists explained the historical theories behind Hanami as a ceremony of fighting gods during the beginning of harvest but now is more ritualistic and festive. It was mentioned that gods and spiritual power reside in plants hence the cherry blossoms. Since Momonga is their equivalent to a god the Floor Guardians decide to perform a Hanami for their master. In order to have the best ceremony, Albedo demanded the best flowers that can be acquired.She enlists the aid of Aura and Mare to for a "Supreme Flower" though the qualities they are searching for are not on the 6th Floor, thus they decide to seek one in the New World. E-Rantel is deemed a center of information to begin their search, thus Narberal Gamma and Slimeko were sent there. At a pub, the two question the adventurers and learn of the existence of the Legendary Tree.
Returning to Nazarick they report their findings to Albedo, who then called a secret meeting of the Floor Guardians. After listening to the stories of the flowers from this tree she deemed it worthy of their Hanami for Momonga. The protagonist, Aura, Cocytus, Slimeko, and Narberal were sent to the forest to survey the location. Aura and Cocytus encountered Hamsuke who was surprised by Aura's visit in the forest and was introduced to the members of her party. Sadly she couldn't help them as she never heard of the Legendary Tree however decided to help them find it. She mentions something strange happening in the forest as goblins and ogres were migrating near her area of the forest.
This indicated that something may have chased them and the group decided to investigate. They encounter the goblins and ogres and after a brief scuffle, question them as to why they migrated. According to them, they were driven by a tree that spontaneously grew and attacked them. Interested, the group ask the ogres to guide them to the tree. There they find a Predatory Plant with moving branches. Aura notices that the surrounding plants are dead and notes that the plant monster is taking all the nutrients in the area. It immediately attacks Aura but she easily dodges the branches but is surprised at its speed. Not only that the branches were now wrong to great proportions. Fending of its attacks and defeating it, Hamsuke believes it to be the Legendary Tree, but the others find its strength to be lack and as well as in flowers. However, it's definitely not a normal plant and must be a cause for its abnormal growth. Taking a sample of the tree for a closer look, it's estimated to be a thousand years old and discover seeds with parasitic traits like mistletoe. They reasoned it was merely a host of seeds that were germinating from the Legendary Tree and take it as a clue. Apparently, this tree was mentioned by adventurers through a legend of a group of heroes that encountered it and promised to slay it if it returned. For some strange reason they heroes they never returned but the plant had continued to grow and kill the surrounding plants. Meaning that the parent tree that spawned the seeds that infected the tree they fought would have incomparable power. It's reasoned that the local contaminants by the Catastrophe may have caused the awakening of the tree. After sending a sample to Nazarick, they decide to search for other regions with dead plants for the possible whereabouts of the Legendary Tree.
Encountering a force of contaminated creatures and Chaos Beasts, Aura estimates them to have come from the east. They come across a deep section of the forest where all the trees were dead. Despite the dreadful aura emitted by the location, the party resolves to claim the Legendary Tree for Momonga and enter. The group spots a living tree with a head full of bloomed flowers and were in awe at its appearance seeing as both terrifying and beautiful. Knowing now they have located the Legendary Tree, Cocytus attempted to cut it down but was resisted by the tree and was attacked. Suddenly the dead trees began to move and uproot themselves, with one of the surviving ogres sided with the trees which in truth are treants that have been zombiefied by the Legendary Tree.
Surrounded the groups' fight, but the trees are endless. Now at a disadvantage, the group decided to retreat. Narberal then used [Electrosphere] to make an opening to escape. Regrouping outside the Forest of Death they plan their next move. In truth, Cocytus could probably win in a one to one battle against the Supreme Odor Flower though the treants are a problem to deal with them interfering in letting Cocytus land a blow against the Legendary Tree. They believe that as long as they defeat the Supreme Odor Flower they will succeed and the zombie treants will return to normal. Aura was tasked to distract the treants to allow Cocytus and Narberal time to fell the Legendary Tree. Returning for another round against the tree and its minions the party succeeds in slaying the Supreme Odor Flower.
It's death coincided with the death of the treants and the recovery of the forest. The group then transported the Supreme Odor Flower corpse to Nazarick on the 6th Floor where they surprised their master with a flower viewing with the plant monster as the center piece. Momonga who is touched by their affections, joins in the celebration and festivities they prepared, strengthening the bond between him and the NPCs.