During a fight in which Sailor Moon, Sailor Mars and Sailor Mercury were pursuing Jadeite, the villain fled through multiple pocket dimensions, but Mercury somehow ended up in a dark place. There she engaged a Horned Reaper who tried to convince her to bond with a dungeon heart (specifically: a sub-heart of Keeper Metriande). After long analysis she tried her own approach that ended with her encasing the Reaper in solid block of ice and her accidentally bonding with the heart. After a few hours of unconsciousness due to the painful soul extraction required for the bond Mercury analysed the heart further and sent the Imps to excavate room for a library. After magically transfiguring raw material into shelves and assorted stuff into a library the Reaper engaged her in combat again after his icy prison had thawed sufficiently for him to break free. They came to an understanding with the Reaper somewhat teaching Mercury in Dungeon Keeping.
Later she left the Reaper in a room with a collapsed entrance and went to the surface. After finding an inn she was quickly found suspicious by the locals and the attending adventurers (Jered, Cathy, Snyder, Boris) transported the arrested senshi to the capital for a trial. In the capital she joined the soldiers attacking Keeper Arachne's dungeon. In the aftermath of the successful attack Ami was found out to be Keeper, but Jered, Cathy and Snyder joined her ranks in turn (reasonable due to their already suspicious association with Ami).
As soon as they were back in their dungeon the impeding danger of an invasion by the baron loomed at the horizon, necessitating plans for a move of the base of operations, either by production or acquisition of other dungeon hearts. The excavation and activation of a portal to the underworld was met with success and the Reaper turning traitor. In the subsequent fight Cathy joined Ami in combat and soon left the Reaper no choice but to flee. 
The information and minions gathered let her attack the weakened Keeper Malleus in the desert with ice golems, with a very narrow defeat of him. After he submitted himself to her control Ami searched his mind for usable knowledge, which made her experience parts of his depravity and brutality. That made her disregard her inhibitions regarding his mental health, ripping the knowledge from his mind. Still in emotional turmoil she killed the gibbering wreck he had become moments later. After a short psychological reconvalence period Ami used the knowledge to fake the destruction of her old dungeon at Goodplace by Baron Leopold.
Successfully relocated in her new Desert Dungeon Ami tried to contact the Light Gods in one of their temples, gaining knowledge about their standing in regards to her and their understandable but unreasonable unwillingness to help her get home. In the process she was - unbeknownst to her at the time - healed of the psychological trauma she endured in Malleus' mind. This contact of the other side angered the Dark Gods who afflicted her and her minions with a plague. Unable to heal it satisfyingly Ami had no other choice to succumb to their demands to a certain degree, resulting in the removal of the plague, having to fight the Horned Reaper in close combat soon and getting her former enemy Jadeite as a new minion.
Due to various circumstances and misunderstandings, many rumours about Mercury are abound, both in the Underworld as well as on the surface.
- The first time an outsider saw the Horned Reaper for decades he was clad in an appropriately customized uniform of Sailor Mercury, including accessories, make-up, skirt and all. Actually the senshi transformation applied the uniforms to every minion of Mercury.
- The naked form of Sailor Mercury was seen straddling the lying Horned Reaper and furiously insulting his inadequacies while at the same time screaming and hitting the big red creature. Actually Ami botched her first attempt at possession spells and switched bodies with the Reaper, who... vented.
- She nakedly battled the Reaper another time on the roof of the light temple in Evercalm. In the process of the fight unlucky remarks by the Reaper as to the possession event made it appear as if she had had intercourse with him, further compounded with her inability to clarify that in the following trial due to the need for keeping her Keeper status secret.
- Senshi Transformation
- Shabon Spray
- Shabon Spray Freezing
- Shabon Spray (corrupted)
- Shabon Spray Freezing (corrupted)
- Ice Golems
- Ice Golems (corrupted)
- Sight of Evil
- Mercury Hand
- Life Drain (based on observations of Jadeite's variation)
- Communication (applied to one eye with triangular gesture)
- Firestorm (self-centered expanding sphere of fire)
- Immunity to fire
- Treats small to medium wounds
- Does not heal illnesses
The first few weeks, Ami was mostly in her Sailor uniform, or monks robes respectively after leaving for the capital. Following that she was versed enough in fabrication spells to manufacture her own clothing, but due to her lack of skill in tailoring that was a rudimentary solution.
After reconnecting the portal in the desert dungeon she acquired a set of clothing more befitting of a Keeper - high-waisted trouser, a blouse, a military-like jacket, all predominantly in black with golden highlights.
- Dungeon at Goodplace - Ami's first dungeon where she met the Reaper and the centre for the first parts of the story. After her participation in the attack of Keeper Arachne's dungeon, her dungeon's location was compromised and Ami had to move to a new location.
- Desert Dungeon - Ami's second dungeon. It formerly belonged to Keeper Malleus, but Ami defeated and subjugated him with tremendous risk to herself. It is located below a desert a few hundred kilometres south-west of her first dungeon.
- West Coast Dungeon - Ami's third dungeon, located on the west coast of the main continent. As such it was surrounded by densely populated areas and a city very close by. Its center was actually below a river upstream of the city. Ami defeated the unnamed Keeper, shut down the Dungeon Heart and moved it to the ocean to reactivate it in an iceberg and transport it to the Avatar Isles.
- Avatar Isles Dungeon(s) - Ami's fourth dungeon.
- Mountain Range Dungeon - Ami's fifth dungeon, Situated in a dwarf kingdom hostile to Mercury. Relocated to when malicious corruption made Avatar Isles unsuitable for habitation.