There are four skulls and each of them has a symbols. This symbols change every game play. Each of those symbols has its location to placed. First location, Spawn area. Second location, Airplane Area. Third location, Docks. last, Operation Room. Each those specific location has a small altar that has a symbol. Pick up the skull with a right symbol and place it on that small altar of your choice. Once you place it, zombie will spawn and this zombie will not going after you. But this will go on the altar to stop the skull to charge up. Protect until all of those zombies disappear meaning its fully charge. Bring it back to where you got the skull first and redo on the three remaining. Note: If you failed you have to start a new round to start charging. But if you still have couple skulls remaining that still not charge, you can do that. Not just only zombies will spawn, trasher and spider will spawn too.
Once you complete all four skulls, Secret door will spawn. Go inside and you will see a skull in the center on that room. Press action button to start the last charge up, but this time around. Gatekeeper will spawn and they will stop. Once its done, You can now pick up the Skull Of Sapwe.