The weapon Alchemy warriors use to fight Homuculus.
A Buso Renkin are weapons formed by Alchemy known as Kakugane. The weapon forms depending on the person personality. Buso Renkin were originally created by the Alchemist Regiment in hopes to create a philosophers stone, the ultimate goal, but are mainly use for the destruction of Homunculus around the world. The Buso Renkin activates and performs based on the person fighting instincts. Someone who thinks of acting and a strong offense would get a weapon meant for attacking, while someone who wishes for defense would get a smaller easier to use weapon.
There are three total Black Kakugane's in the world. The first Black Kakugane is placed in Victor's chest to keep him alive, but reacts terrible and turns him into a being superior of that of Homunculus. The same thing happens to Kazuki a hundred years later. The Black Kakugane not only gives the user the super human strength and the ability to heal super fast, but it also comes with the ability Energy Drain. This ability is no different from breathing, something they can't shut off and drains the life of living this around them. This makes the a huge threat as just walking by people could kill them.
There are three stages for a person who has a Black Kakugane. The first stage allows the person to switch back and forth from there Human Form and there "Victor" form. However after six weeks the second stage takes effect and the person becomes a full time "Victor". Stage 3 is nothing more than a powered up stage two, but the Energy Drain ability becomes much more powerful and all that more dangerous.
The White Kakugane was developed by Alexandria, wife to victor, in the hopes that it would negate the effects of the Black Kakugane. It was developed using the third Black Kakugane, and was giving to Kazuki by Alexandria to use as how he saw fit. Kazuki used it on Victor, but because Victor was in stage three all it did was revert him back to stage two. However thanks to the effort of Chouno, the white Kakugane was able to be rebuilt and use to restore both Kazuki and Victor back to normal humans.