The Hamelin family

[Richard Hamelin]

Much like other noble families, the Hamelin are characterized by the fact that their very peculiar Semblance is hereditary.
This Semblance allow them, as long as they are playing an instrument, to hypnotize and submit to their wills low to low-mid level Grimms, however only two or three at the same time per player.
Playing together allow them to seize control of much more of them, but they are unable, sadly, to take control of stronger ones, though those can still be influenced in some minor ways.
Frustrated by this failure, ancient members of the family resorted to experimenting on the Creatures, hoping to maybe find a way to reinforce the effectiveness of their Semblance.
Grimms tend to die in captivity, but they managed to keep them alive by simply using their Semblance to order them to not die. Experiments showed that even stronger Grimms such as Nevermores or Death Stalkers will eventually succumb to their influence after a long enough exposure, though this is mesured in weeks and sometimes months.
An unexpected side-effect, however, is that the captive Grimms, after being exposed to the family's Semblance for generations, started to mutate and evolve, or maybe devolve, becoming much more sensitive to the mind-control, and continuing to obey previously given orders even after the music stopped.
Strangely enough, all the Grimms that mutated in such way took the appearance of a rat, regardless of their original species. This new, weak and easily manipulable species was named after the family that created them, the Hamelins.
There are almost none in the wild, since they do not naturally appear, but if a member of the family falls in battle, then the familiar will be freed, and eventually get rid of the influence that modified their actions.
They can become extremely dangerous due to another battle tactic of the Hamelin family :
They discovered, during the experiments, that they are also able to manipulate the body of dying Grimms. Rather than merely letting them disintegrate, they can stabilize the disappearing body into a sort of sand/cinder like state.
By manipulating said cinders, they are able to recreate full Grimms, although those still are dead and would fall back into dust the moment they stop playing, and are very aura-consuming. However, by using their familiar as a sort of 'core', attaching them to the newly formed body, it will remain 'alive', able to function independently and still obey them loyally.
To come back to the topic of freed Hamelins (the Grimm. Most people agree that naming them the same as their family was stupid), they would find themselves able to devour other Grimms, growing in mass and strength with no theoretical limits.
Thankfully, they are either always retrieved or dispatched before they are allowed to developp too much.
Still, the Hamelin family, even todays, keeps experimenting on Grimms, believing that within their Semblance lies the key to finally free humanity from the curse that is the Creatures of Grimm, and they will persevere, no matter the cost.