There are lockets and then there are Lockets. The one of interest to us here is the gadget that was upon Agatha Heterodyne by her uncle Barry when she was a child. It was imbued with the power to suppress her Spark by inducing painful migraines whenever she attempted to enter the madness place, breaking her concentration and forcing her to behave "normally". While it is an issue that is made more explicitly clear in the novelizations of the strip, it also had the effect of damping down all of Agatha's emotions, both positive and negative.
This may seem like a nasty thing to do for one's beloved niece, but there was method to Uncle Barry's madness. Many Sparks die young as a result of building their first creation -- a process known as "breaking through" -- before they have the wisdom to control that which they create. As a result, either their clanks kill them, or the enemies they turn the clank against do. Presumably, Barry didn't want Agatha to start acting "Sparky" until she was old enough to deal with whatever she made the first time out. (It's also been commented that female Sparks in Europa have a nasty habit of simply disappearing...) It's further noted in the novels that the device was probably intended to be a temporary fix, but then Barry went away on his mysterious trip, from which he never returned.
We have some idea what the device's innards look like, including portraits of Lucrezia Mongfish Heterodyne and Bill Heterodyne. It was in the cover of Issue 10 of the Comic Book, and also currently appears as part of studiofoglio.com's Free Stuff.
Lo and behold, this crazy idea worked. Agatha survived and kept the Spark-suppressing locket on until the age of eighteen, when our story begins and she is Beetlesburg by the von Zinzer boys. Omar steals it from her, and it appears that the locket soon kills him, the most obvious theory being that as a non-Spark, he has various important bits of his brain tamped down. An engaged Moloch then the device, but retains it on his person as he seeks out vengeance, thinking that the death was deliberate on Agatha's part. Meanwhile Agatha's Spark almost immediately makes itself known, and her first clank follows shortly, with no significant ill effects, and an awesomely powerful Spark is born.in an alley in
Moloch retains the locket after he and Agatha get carted off to Castle Wulfenbach, whereupon he to her, but refuses to hand it over, seeing it as a bargaining chip in their relationship. After Moloch's attempted Spark-impersonation fails and Agatha escapes the airship, the locket is confiscated by Klaus Wulfenbach; he's seen silently In the print-novel version of this scene, Klaus correctly IDs the device as being Barry's work, and mentally describes it as being the most sophisticated thing his former friend has ever made. He also deduces it operates by producing some sort of tone or signal using miniature tuning fork-like components.
It's never stated exactly what happens next, even in the print novels, but it appears that Klaus repairs the device, and he has it in his possession when he finally confronts the Lucrezia-possessed Agatha in Sturmhalten. Lu finds it after successfully wasping the Baron, and puts it on, thinking it is only decorative. Unfortunately for her, it immediately putting Agatha back in charge. Even better, now Agatha's mind has developed to where she can do Sparky things , so she kinda gets the best of both worlds. Woe be to us all, however, whenever the locket gets removed. Zeetha, who knows Agatha well by way of Kolee-dok-Zumil, Agatha the first time she starts to remove it, but it later gets while Agatha is in Castle Heterodyne. Fortunately, Lucrezia is (mentally) chased off by Von Pinn and Agatha herself, though not before a copy of her gets put inside Zola. The locket also gets temporarily in the Fortress-Depot of the Corbettite Monks, but this time around Lucrezia succumbs to some drugged cake, and Agatha wakes up back in charge again.
The End (For Now)
Unfortunately, as Agatha's adventures continue, it gradually becomes apparent that the Lucrezia-copy is also developing a resistance to the locket's power, and she begins to "bleed through", influencing Agatha's actions; one notable instance of this occurs in the Immortal Library under Paris, where she Agatha into freeing the trapped Muse of Time. And then finally, in the Londinium undersea dome of the Queen's Society, Lucrezia takes full control of Agatha's body even with the locket in place, realizes how to ascend to "second-stage" Sparkhood, and does so, becoming a glowing giantess. As part of her triumphant rant celebrating this event, she strips the locket off her neck and , presumably destroying it. (Said triumph is short-lived, but that's a discussion for elsewhere..)
In The Future, Agatha has been glimpsed wearing a locket that appears identical to the destroyed one, except for the addition of a pair of wings. The significance of this remains to be seen. An interesting note is the Wulfenbach Sigil is a castle with wings.
For speculation and discussion, see Agatha's Locket on the forum.