One of the dao cultivators of the earliest time Li Shen had planted a tree in heaven to balance the energy of ying and yang.  There are three worlds, the heaven where immortal beings live, the earth where humans live, and the underworld where demons live. The tree nourishes itself from the positive and negative energy human beings emit from their positive and negative emotions. It bears two kinds of fruits because of absorbing two kinds of energy, one is of violet color, and when it ripens little fairies are born and the other kind of fruit is of charcoal black color, on its ripening dwarf demons are born that then goes into the underworld. The tree looks half shrouded in darkness and half emitting light. That's why it was named 'Ying Yang Tree'.

The light fairies after aborning went into the human world and there they are the source of positive emotions humans carry and from the underworld demons somehow went into the human world and became the source of negative emotions. This is the cycle that keeps the positive and negative energies of the three realms under balance. If somehow there is an imbalance, then it could disturb the whole cycle and it could lead to chaos into three realms.