The iconic keyblade found in KH3's trailer; surely it will be integral to the final chapter of the SoD Chronicles.
Originally belonging to Eraqus, Master Keeper is a special keyblade with ties to LoD. As a defense mechanism against the forces of darkness, this keyblade can transform LoD in CO, thus creating the Chamber of Waking. However, only Aqua can find the CoW, probably because she has Master Keeper.
Keep in mind that keyblades of heart only existed as a result of the universe's reorganization. The Keyblade of Peoples' Heart, the only known keyblade of heart, came into existence as the key to unlocking the DtD. Not only does it release the human heart to darkness, but it also releases the RoL to darkness. If the category of "keyblade of heart" can be interpreted as a medium between light and darkness, perhaps there exists a reciprocal?
A reciprocal such as Master Keeper.
LoD is a world that's supposedly significant to the balance between light and darkness. If darkness were close to overcoming it, MK would be used as a failsafe to prevent that. Not to mention, it continued to exist after Eraqus' death, so its continuous existence could be seen as an opposite to the KoPH's lack of continuous existence.
Whereas the KoPH exists to help drown the universe in darkness, MK would exist the keep the light shining.