Horologium Chamber is a legendary chamber accessible through a secret passage hidden behind one of Luna Nova Library's bookshelves. Its main room is a spacious and elegant hall located at the end of a series of descending stairs with a sofa, shelves for containing several relics/items, a desk, and torches for illumination. It has two sub-entries; one that leads to a room containing Horologium Clock and the other is a magical door that can connect to various places depending on the magic key that is used to open it, though it all pales in comparison of what truly lies on the other side.
Horologium ClockLocated on the right after the entrance is a large room that contains the Horologium Clock, a huge clock in which needles always move in the opposite direction. It consists of numerous gears and multiple octahedron crystals as pendulum surrounded by a large magic circle. The Clock also contains a seemingly limitless amount of Time Magic which allows those who tap on it the ability to rewind time and even create the endless time loop with only the user and those who have entered the Chamber who can sense it. However, for the same reason, Horologium Clock is the known source of Curse of Time, a malediction where the afflicted got caught outside the flow of time and gradually corrupted as a result.
When Akko and co. entered the Horologium Chamber for the first time, the Clock appeared to be broken until Constanze restored it with use of Elder Treant's sap. The damage is implied to be the result of Molly's attempts to use its power to go back in time and one of possible reasons of her being afflicted with Curse of Time.
Sealed DoorThe enchanted door which connects to 7 different dungeons accessible with their respective Magic Key. The Sealed Door also has a magic glyph drawn in front of it, which designed to prevent more than three people from crossing it safely as well as instantly teleport them back to the Chamber if needed. During Akko, Lotte, and Sucy's first exploration into one of the Door's dungeons, the glyph was revealed to be broken, yet it miraculously managed to bring them back despite its condition. Diana would later restore the glyph to prevent future incident shortly after getting caught into their adventures along with Amanda, Constanze, and Jasminka.
The following are the list of dungeons accessible through the Sealed Door and their respective Keys:
- Canopus Ruins – Canopus Key
- Mimosa Forest – Mimosa Key
- Mountain Ankaa – Ankaa Key
- Regulus Mine – Regulus Key
- Shaula Desert – Shaula Key
- Gienah Tundra – Gienah Key
- Labyrinth of Pollux – Pollux Key
Time VoidThe true interior of the other side of the Sealed Door. It is a dark void which illuminated by what appeared to be stars and has several floating ornate circles that can be seen everywhere as well as a large rock to stand on. This is where the seventh fragment of Molly's Ghost Witch half locked within before eventually reunified with the rest only to be defeated by Akko and co. in an epic battle and purged along with Curse of Time that created it.
Horologium Chamber is a secret chamber that had been hidden in Luna Nova Library since ancient times. It was built either during or sometime after Luna Nova's construction, specifically to contain Horologium Clock due to its dangerous properties.
The Chamber remained locked and undisturbed until Golden Age of Magic era where it was discovered by a Luna Nova student named Molly. Discovering the time-manipulating power inside, she harnessed it in order to improve her initially poor magical skills. As much as she was happy no one made fun of her anymore thanks to Horologium Clock's magic, Molly's inability to reveal her secrets to everyone made her isolated herself from others and became lonely as result. Seeking to change that, she channeled the Clock's magic to return to the day she first attended Luna Nova. But instead, she was afflicted with the Curse of Time, resulting her to be re-enrolled in Luna Nova each year only for everyone to forget about her on the following year. To make matter worse, the curse eventually corrupted her into a skeletal monster labelled as Ghost Witch which then wreaked havoc on the academy.
In response of this, the professors of Luna Nova at that time stopped the transformed Molly's rampage before separated her corrupted half from her human half and splitting it into 7 fragments; the seventh fragment was locked within the Time Void behind Horologium Chamber's Sealed Door, whereas the rest in her Classic Witch Set. After hidden the set's pieces in six of Sealed Door's dungeons, they placed a magic seal over the Horologium Chamber to both keep her splintered Ghost Witch half trapped and erode the curse until she returned to normal. The same seal also has a failsafe in form of a time loop which activates should either the seal or the Chamber got disturbed before it could complete its purpose. In that meantime, Molly's human half continued attending Luna Nova with memories pertaining her in everyone periodically reset once a year as before, but this time her own memories also reset as well.
In the present day, when Akko is sleeping in the middle of her punishment at the library, Molly's human half, finally remembered a bit of who she was thanks to the seal's hold over her corrupted half's seventh fragment has finally weakened, left behind a book with the power to undo the seal of Horologium Chamber near her. This led to the Japanese young witch, alongside her roommates Lotte and Sucy to unlock Horologium Chamber once again. From the very moment they stepped into Horologium Chamber and even ventured through one of its seven dungeons, the three friends unknowingly triggered the seal's failsafe, resulting them and later, Amanda, Constanze, Jasminka and Diana to be trapped in a time loop, repeating the first day of summer holiday. The gang attempted to break the loop to no avail and subsequently investigate the connection between the Seven Wonders and Magic Keys they found on each location where a Wonder took place for answers. The gang also retrieved Clasic Witch Set from their explorations at the Chamber's dungeons, unknowingly bringing 6 of overall fragments of Molly's Ghost Witch half within back to Luna Nova and allowed them to reunite with her human half.
While investigating the final Wonder of the Seven Wonders of Luna Nova, the ghost in the library, Akko meets and befriends Molly who curious with the Wonders. However, it soon comes to light for the Japanese that Molly is in fact, the Ghost Witch and the one who halted their progress in breaking the time loop all this time. To make matter worse, she now has all fragments of her Ghost Witch half minus the seventh one in Time Void behind the Sealed Door of Horologium Chamber. She tries to attack Akko after revealing her true colors only to be forced to retreat thanks to timed intervention from Ursula's, but not before scattered Akko's collection of Chariot cards both to cover her escape and to make Akko feels the same despair she had. After retrieving scattered cards and putting pieces together with Ursula's help, Akko and co. confront the newly reunified Ghost Witch, defeating the beast and successfully break the Curse of Time, restoring Molly back to normal. With Golden Magic Age professors' seal over Horologium Chamber has completed its purpose and the Ghost Witch is gone, the time loop which trapped the group finally ceased.
Afterwards, Ursula sealed the Horologium Chamber once again to prevent similar incident from 200 years ago from repeating itself.