This Skull keycap is made by the Chinese artist ‘Hammer’. Keycaps are available in limited quantities on Imsto’s website. They are available with either MX or Topre Mounts. Solid Colour keys are made to allow light to shine through the eyes. More information can be found in Imsto’s thread on Geekhack.
I believe Feng on Geekhack also ran a group buy for Titanium versions of this keycap before they were made out of resin. I couldn’t find the original link to this group buy.