Grand Design

The grand design is what the higher beings in the world created. there is a certain way the world was meant to stay. in one episode with the battle with Gideon, there was a mirror that allowed Gideon to look into the mirror world of the one they were in. in their world good was the prevailing force and in the the other was evil. while trying to send Chris and Leo back to Chris's time with the help of Gideon, they are "accidentally" sent to switch places with the evil persona from the opposite world. Gideon trying get the sisters separated from wyatt to kill him, Gideon said that it was a part of the Grand Design that there be balance within the worlds. the problem was that the evil Chris and Leo comitted a great evil in their world and Gideon sent Phoebe and Paige to the other world to get the good chris and leo. this in turn created an imbalance in the grand design and cause reality to shift. to correct this, leo had to do a great evil on his side of the grand design. and so he killed gideon, the elder.