Inheriting a legacy may seem good, but it can be a mixed bag, depending.

Inheriting your father's estate, resources, riches, and preternatural talent for dark magic is good.
Inheriting your father's enemies, tendency to make them, and embarrassing surname, however, is not so good.

In the case of Mortimer Shadowbane, this meant he needed to cover as many bases as he could to better utilize the mere half of the magic power he received from his patriarch.
Thankfully, Blake Shadowbane had hoarded many ill-gotten tomes and equipment which were now the property of his son.

Through his self-teaching, Mortimer found that the most efficient way to protect himself from what Blake left behind and attain power would be to gather as many cronies as he could.
To avoid spreading his resources thin, he got creative and explored many avenues:
Summoning extraplanar bondservants, creating animated constructs, raising undead corpses, enchanting, tricking or even bribing creatures into his service.

One such member of this menagerie was a servant created late-term, not so much out of necessity but of convenience. 
After coming into possession of a bigger stockpile of mercury than he could ever use, the shady mage decided to experiment and bring it to life.
Though it was a success, the creator was already disinterested, having no clear use for this creature.
"Stay in this old cauldron out of the way, kill any intruders that enter the laboratory, but make sure nothing ever gets broken." Was the only directive the confused soul ever got. Not even a name to go by.
In this way, the laboratory was their post and also their prison. They would only ever see Mortimer as he used the laboratory, though he never spoke to them, and sometimes a guard quickly peeking in during their rounds.
It was a mind-numbingly lonely existence, looking at these countless bottles, vials and canisters from the rim of their cauldron with no concept of night and day to mark the time.

Until one day salvation came in the form of Mortimer entering the room accompanied by a hauntingly beautiful woman.
She was given a quick tour of the premises before being left alone to start her work, which seemed to involve filling the empty pages of a tome.
It took her no time to detect the cauldron-dweller's presence, or perhaps she even knew all along, because she called out to them barely a few minutes in.
This was the first time someone addressed them since Mortimer's first and only words, to say nothing of the fact they were actually invited to respond, this time. It was at this very moment that the mercurial being discovering they were even capable of speech.
The mysterious woman introduced herself, her name was Bel. When no name was available in exchange, Bel offered one: Mercy. A cheap play on the word mercury to be used as a placeholder, but little did she know at the moment that Mercy would happily use it forever.
They kept each other company while Bel worked on the book. She explained that she had Mortimer host her for a month as an exchange of favours, her end of the deal being to write down all her available knowledge on medicines.
Mercy had little to offer in lieu of conversation, having limited experiences, but they drank any stories that Bel volunteered like life-giving water.
This companionship had an interesting side effect, Bel being literally the first role-model Mercy ever had access to, the former began to imprint on the latter. 
After a few days of being visited, Mercy had adopted the same sultry tone of voice as Bel and made themselves to look like a similarly curvaceous woman. Though surely more exaggerated.

Bel thought this was endearing. Though she saw a more worrisome pattern to this behavior, which she made sure to bring up to the master of the house the next time he was in the room.
She admonished him for not allowing Mercy to socialize with her peers, vouching for her feelings, which he had surely not noticed when creating her.
Mortimer was quick to retort that his creation required no socializing, and that no matter what personal will it may have, it could not stand up to his own.
He left Bel with a warning to not abuse the privilege he granted her within his domain before storming back out of the laboratory.

At this point, the devilish woman took it personally, and as soon as Mortimer had business outside the tower, she sprung into action.
She invited Mercy to follow her outside of the laboratory. Mercy, who had only ever known to stay inside that very room, suddenly felt the geas holding her drop in priority. She hesitantly followed her friend through the door, finding it to not only be possible, but also easy, and into a much larger world: the hallway.
She knew the world was larger than that laboratory, but she had never been able to experience it for herself, and actually being in a different room felt more exhilarating to her than she had thought it possible.
She followed Bel on her stroll through the tower's halls, gliding over the flagstone, and soon her mind got opened rapidly in a new way. She started meeting other inhabitants.
So many of the things she had heard or felt from the confines of her prison were suddenly put into context: The guard occasionally peeking in everyday was an orc from a hired squad, the thunderous footsteps coming ever so often from beyond the door were of a mindless minotaur patrolling the halls, the infrequent crazed squabbling came from a gang of goblins that were kept hooked on a substance that Mortimer produced in the infamous laboratory.
These encounters multiplied during the day, her novice conversation skills were put to a harsh test, but despite this, the disposition she inherited from Bel made her quite popular amongst the mostly male personnel. Many were eager to flirt with her, which gave Bel a lot of work in making sure no physical contact happened and explaining many times in the day how Mercy's body is, sadly, very toxic to the touch.

This clandestine ritual happened many more times, whenever the Master's absence permitted it. In hindsight, these days of making friends and acquaintances by her own capacity were the best of her life so far.
But sadly they could not continue forever. The inevitable eventually happened as Mortimer caught wind of this misbehaviour and went for a confrontation, surprising them in the very act.
The resulting shouting match between the sorcerer and his guest attracted the curiosity of a handful of witnesses.
Mortimer was making a point of saying that Bel had vastly overstepped her boundaries by giving Mercy, who was cowering behind her friend the whole time, such freedoms.
Bel snidely quipped back that she literally could not act outside the licence given to her even if she wanted to, and that it was his fault for wording their agreement so vaguely. To say nothing of the fact that it was only a verbal contract.
This made the grizzled mage furious, for he knew she was right. The clause of allowing her to "have her way" with his minions during her stay permitted her much more than what he assumed she meant by it.
He felt humiliated, and in a moment of uncharacteristic emotionality, he decided he would not let her have the last word. Without warning, he shattered the summoning spell that kept Bel anchored to the material word, thereby breaking their agreement by sending her back sooner than the settled upon time.
As a rift in dimensions opened at her feet to forcibly drag her back to the netherworld, Bel let out a hellish voice cursing Mortimer's name, and promising that he'd come to regret this decision.

When Bel disappeared for good, something clicked in Mercy's mind, toggling it back to how it previously was, and she simply started heading back to her cauldron of her own initiative, or lack thereof. Meanwhile Mortimer barked at any nosy bystanders to return to their posts.
While her mind retained everything it had gained from these interactions, Mercy was woefully back to her personal hell of being made tranquil by her master's will as her creator.
Once more, days turned into weeks, weeks turned into months, and months turned into years. 
But at least not everything was back to how it was, the other denizens of the tower were now aware of her existence. Rarely, those brave and curious few who could attempt it came to see her, providing some much needed company and mental stimulation.

A few years passed in this fashion. Until a fateful day came that would change the lives of everyone in the tower.
The way Mercy experienced this pivotal event was that from one moment to the next, she suddenly felt freedom. But more importantly, she no longer felt any obligation to abide by Mortimer's word.
She immediately exited the cauldron, tasting a range of movement she had dearly missed, and wandered about the laboratory in a confused daze. She could feel a strange buzz going through the whole tower. Until finally someone, a guard, came crashing through the door to check in on her.
The entire tower had been cast into chaos, the orc explained as best as he could that many creatures suddenly disappeared into thin air, that many others became completely unhinged and started wreaking havoc, and that the chain of command was broken among those of sane mind.
Those clever enough to assess the situation and put the pieces together knew what this meant, but in the current tumult it would be hard to spread the important news that, verily, Mortimer Shadowbane the Sorcerer had passed away.

It took until sundown for a semblance of control to be regained over the late Mortimer's fractured estate. Confused beings back in control of their senses had to be soothed, mindless creatures with no one left to control them had to be fought back, insurgent individuals with uncertain futures had to be talked down.
Many lives were lost in the struggle, and what was left in the end was a sizeable group of survivors in an awkward makeshift alliance.
A non-negligible stake of real estate had just been freed up with no clear next of kin to claim it. And an even more significant wealth of goods stored inside.
Though many had thought it, nobody had the individual strength to conquer the tower against the combined might of the rest. So a begrudging accord to work together and coexist was born.
But the uneasiness of their pact would be short lived, as they'd find lots of common ground in their mutual dislike of their former employer. 

A majority decision was taken to start selling off most of the equipment and supplies for currency as none of those remaining had any talent or interest in the magical arts. This money would help many rebuild their lives, and for others to get theirs started.
Cataloging and appraising everything was a daunting task, but everybody willing pitched in. Mercy was given duty of the goods of the laboratory, as she was deemed the most familiar with it, if only by a technicality. Thankfully, the ex-owner kept many notes which helped cross reference almost everything.
Before long, potential buyers started visiting the strange yard sale. Many of these were people with enmity towards Mortimer, perhaps they were the first to catch wind of his demise. The rest were mostly practitioners of the arcane, looking to make good on an opportunity.
The money slowly trickled in despite the inherent tensions of allegedly civilised races bartering with monstrous beings. As space freed up, the found family were able to make a home for themselves in the keep.
They never learned who or what it was that killed Mortimer, in the end, nor did they care. For many of them this was a windfall, and for others, it was true freedom.
Though this job made her spend a lot of time in the lab, Mercy found it therapeutic to be making a living off of liquidating the assets of her ex-captor.
The very first thing she bought with her very own money from one of the traders that made trips to the tower, was a huge, 20 gallon, glass carboy. She abandoned the blackened, sooty cauldron she was forced in for the beauty and softness of forest glass. As a bonus, the rope lattice that wrapped it protected it, meaning she could safely roll it from one place to another. 

This new, personal vessel would be the symbol of her new life. The seemingly mundane act of making purchases for oneself would be one of many more to follow, as Mercy continued to explore the means at her disposal to live a comfy life of peace, while navigating the unique existence of being a woman of mercury, with her new friends and family.