The Sealed Evil Tree is a CD drama story of the Overlord series. It was released alongside Overlord Volume 04.
Momon has been tasked by the Adventurer's Guild in retrieving a rare herb deep in the Great Forest of Tob. But what was supposed to be a simple retrieval mission takes on a path that leads Ainz Ooal Gown to an unexpected situation.
Pluton Ainzach invites Momon into his office for a personal request. Theo Rakheshir is also present in the room and welcomes the adamantite adventurer. Surprised by the presence of two guildmasters, Momon feels honored, and continues to be courteous despite the attempts of the older men to relax. Ainzach has a request to retrieve a rare herb grown in the northern region of the Great Forest of Tob. Momon happily accepts the request, though Ainzach warns him that the task might be too difficult. Thirty years ago, a team of adamantite adventurers, along with two teams of mithril adventurers, undertook the quest, underlying the seriousness of the situation.
Despite their protests, Momon agrees to the request and decides to prepare to embark on the mission. After the adventurer leaves, both Ainzach and Rakheshir have a discussion on Momon's seemingly selfless personality. Ainzach laments at the day when Momon may leave to seek out the last of the two vampires he was tracking. Since Momon had already killed Honyopenyoko, Ainzach wished to prevent him from leaving the city. He has already made preparations to hire several prostitutes to sleep with Momon and hopefully bare his child.
On the topic of Momon's origins Rakheshir believes him to be a former noble, as evidenced by his high quality gear and magical items in his possession, and believes that he traveled to the Re-Estize Kingdom to slay the pair of vampires that destroyed his kingdom. He also has reason to believe that Nabe is some sort of relative of his or servant, explaining her loyalty. Ainzach worries at the thought of the two of them being lovers. Unsure of the adamantite adaventurer's origins, the two veterans agree to keep their thoughts to themselves to avoid offending the hero.
The scene switches to Ainz, traveling on the back of Hamsuke. Passing by Carne Village at a distance he muses that the construction of the fortifications seems to be going smoothly thanks to the golems he lent. Hamsuke notices Ainz dropping a small wooden doll on the way. Ainz tells her to pay it no heed as it will be used in case they need to teleport back to their present location after they find the herb in the Great Forest of Tob. Entering the forest, they are greeted by Aura Bella Fiora, arriving from the base inside the forest. Ainz informs the 6th Floor Guardian of their mission from the Adventurer's Guild. Aura knows about the location where the herb is located, however she had never seen such a herb growing there, only dead vegetation.
The trio decides to investigate, with Aura riding upon Hamsuke and using her ability to temporarily boost Hamsuke's stats to make her go faster. By nightfall the trio find themselves in a cleared part of the forest. Aura can provide no clear explanation as whether it was natural or artificial. Ainz then decides to set up camp for the night. For shelter Ainz uses Green Secret House to build a temporary lodging for them. Just as they go inside, Aura and Ainz begin a discussion on ways to improve Hamsuke’s levels. However, Aura detects a presence right outside their shelter. She notes that the presence was able to hide from her abilities, and warns her master that it could be dangerous. Ainz, wishing to investigate what was outside, uses Eyeball Corpse and sees that it is merely a dryad.
While so, Ainz and Aura appear outside and express their peaceful intentions, prompting the forest sprite to appear. The dryad introduces herself as Pinison Pol Perlia. The dryad thought they were the people she made a promise to long ago. As it turns out a dangerous treant, Zy'tl Q'ae is starting to awaken. The lack of animal and plant life in the area is a sign of the monster sucking all the energy in the forest. The people Pinison met in the past were able to seal a part of its body, but now it will soon fully awaken. The dryad is disheartened that those that saved her broke their promise. Ainz, interested in the creature, offers to save Pinison, in exchange for becoming his underling. Pinison is hesitant for such a commitment, but Ainz blackmails her into accepting, stating he would leave her behind. The dryad accepts his proposal, and Ainz reveals his true form to her as an Overlord of the Great Tomb of Nazarick.
The dryad takes the three to where the treant lies. While debating on how to attract the monster's attention, Zy'tl Q'ae reveals itself. Upon awakening, it immediately starts to use its tentacles limbs, ripping great trees from their roots and devouring them. While the treant goes about feasting it takes notice of the intruders in its domain. Aura then notices the herb on its head.
The Floor Guardians then arrive at Ainz's command. Finding the constant roar annoying as they are holding a conversation about what to do with Zy'tl Q'ae, Demiurge uses magic to freeze the great tree in ice. Ainz orders the Floor Guardians to act as a team to fight against the treant as a form of training and to also retrieve the herb for him.
Though incapacitated, Zy'tl Q'ae manages to break free from its entrapment, more furious than ever. Turning its sole attention to the Guardians, the treant attempts to crush them with its tree limbs.
The Floor Guardians carefully fend off the treant's attacks through a combination of their strength and teamwork. Eventually Ainz steps in, satisfied with their coordination; he freezes Zy'tl Q'ae in time, decorating its trunk with numerous vessels of exploding spheres, making the fearsome treant look like a Christmas tree, before exploding.
The last scene has Ainzach greeting Momon, stating has already assembled three teams consisting of one Mithril and two Platinum ranked adventurer teams. When he lays out his plans to Momon he is ultimately shocked when he is told that the request has already been completed in those three days since Momon has departed and drops the requested item on his desk. He is last heard shouting in disbelief when Momon exits the room.
- The events of the CD drama take place just before the events of Volume 04.