From Introduction:
This creature resembles a humanoid skull with a single gemstone embedded into each eye socket.
Screaming skulls are immobile guardians created with the purpose of guarding or protecting a specific object or area. They are often used in concert with other monsters as their screaming alerts other creatures to the presence of intruders. A screaming skull obeys its commands to the letter. Should its creator die or should the screaming skull be unable to carry out its commands, its magical nature ceases to function and it becomes nothing more than a normal skull. Its gemstone eyes dull to a lackluster gray (though they retain their value). Combat When a living creature moves within 40 feet of a screaming skull, it unleashes its hellish scream. The screaming skull continues its assault until it is destroyed or its foes leave the area. The gemstones that function as the screaming skull’s eyes can be pried from a destroyed skull by succeeding on a DC 20 Strength check.