When your hero reaches level 40, Zodiac will be opened. There are 5 Zodiac plates (Blue Dragon, Phoenix, etc.) unlocked. You can use Empire points to unlock the stars on each plate so as to enter the next system.
The Zodiac system can provide Stat bonus to your hero and pet. Meanwhile you can choose a pet (equal to or above level 30 rare pet) as the Soul Guard and some Stat of the Soul Guard pet can be added to other pets you have.
The more stars in Zodiac system are unlocked, the higher Stat bonus your hero will obtain. The better the pet as Soul Guard is, the higher Stat bonus the other pets will obtain.