Dead Island Purple Skull Location: Developer's no. 4 Craft Mod
Dead Island has a number of various secret colored skulls scattered throughout the island of Banoi. There are at least 4 confirmed skulls: orange, brown, purple, and blue. However, there may be more than just those four on the island. Finding these skulls and dropping them off at specific locations rewards you with a secret weapon mod. These weapon mods are very expensive to make at a workbench, but they are better than normal weapon mods. Also, I have heard some speculation that you may be able to combine all the weapons to create a "super weapon." (This is not confirmed, this could prove false.)
Purple Skull location in Dead Island
This skull can be found in the sewers underneath the city of moresby. It is in the room with toxic barrels. There is a ladder that leads to a platform. Walk around the corner until you see a destroyed mattress & a weapons crate with the purple skull next to normal looking skulls. The words "it's our own fault" are written above it on the wall.
To make this room easier to find, look for a floater zombie enemy type. He will likely be beside the toxic barrels and the ladder leading up to the purple skull. A floater is the zombie that appears bloated & fat and vomits on you. They are difficult to kill quickly.
Do you think the developer's no.4 craft is too expensive?
Purple Skull Drop Off Point for Developers no.4 craft mod
In order to get the weapon mod, you need to go to the apartment building where a woman is screaming for help outside her apartment in the city of Moresby. This leads to a mission where you need to kill all the zombies inside. Once completed, she will be on the top floor. In one of the side rooms is a stove. Place the purple skull on the stove and you will be rewarded with the developer's no.4 craft weapon mod.
This mod requires 5 diamonds and one battery. The cost is 10,000 dollars (very expensive). You can create a special explosive knife that kills zombies instantly with it. This is a must have for any character, but for Logan especially, since he is good at throwing weapons.