Mother Core is a power in Battle Spirits Shounen Gekiha Dan .
Mother Core was beared by Kajitsu, the reincarnation of an otherworld Princess. Otherworld King wanted Mother core for his own, and after some time, in a surprise attack on the Momose siblings was able to acquire it, killing Kajitsu in the process.Dan challenged Otherworld King for the Mother Core. He is successful, but is unable to bear Mother core's power. Magisa was then forced to absorb it, and used it's power to separate Otherworld and Earth, and made entry to either world impossible.
Mother Core holds the Otherworld together. It has been shown that it can restore burnt forests, make plants grow on the desert again and generally help Earth. However it all depends on the one holding Mother Core-when Otherworld King took hold of it, Mother Core was contaminated with Otherworld King's insanity and thus caused damage instead.
It should be noted that when Kajitsu has Mother core, whenever Yuuki battles, he feels no pain whenever he loses a life.