Minecraft wiki entry for Sculk_Catalyst, associated text: If a mob dies and a charge is detected more than 4 blocks from a catalyst, a sculk block has a chance to use 10 experience to grow a sculk sensor or a sculk shrieker. A sculk sensor has a 9% chance to spawn and a sculk shrieker has a 1% chance to spawn. However, sculk sensors and sculk shriekers cannot spawn if there are 2 or more sensors within a 9×3×9 box with the bottom center of the box centered on the bubbling charge location (or within -4,0,-4 and 4,2,4 in relative coordinates). The Ender Dragon is the only mob that the sculk catalyst doesn't detect.