Minecraft wiki entry for Development_versions, associated text: Generates in the Deep Dark biome.
Has two soul-shaped patterns in the center.
Has the ability to "shriek", emitting ring-like/sonic particles.
Is activated by a sculk sensor, redstone signal or player's stepping.
Gives players the Darkness effect when activated.
If activated three times, a warden is summoned from the ground.
Sculk shriekers summon wardens only if there isn't one within 48 blocks.