Sighted most recently in Minecraft LIVE's worldwide reveal stream of The Wild Update, the sculk shrieker blocks are one of many new sculk-based blocks found in the deep dark biome and its subterranean cities.
Sculk shriekers, which can currently be found within Minecraft: Bedrock Edition's Creative Inventory (allegedly when experimental gameplay is toggled on or while playing the version's 1.18 development build), are not yet fully implemented and will be made available when The Wild Update goes live in 2022.
However, there's enough information to gather from what has been released on sculk and the deep dark biome to make some conclusions, and Mojang will likely release more info as the update gets closer.
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Minecraft: What we know about sculk shriekers
Sculk shriekers will be found within the subterranean deep dark biome when fully implemented into Minecraft. They can be broken by any tool in the game, though farming hoes will break the blocks in the fastest manner. They retain a mix of sculk's black and cyan coloration while also featuring bone white parts of its texture.
Like many other sculk blocks, sculk shriekers are sensitive to vibrations and sounds, as entities are capable of sending them through sculk growth and into other blocks.
When sculk shriekers are disturbed in Minecraft, either by vibrations, sound, or a redstone signal, they emit a "shrieking" loud noise and create a pulsing darkness that envelops the area.
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The light will fade in and out for approximately 30 seconds before reverting back to normal. It is not yet clear as to whether mobs can spawn in the darkness pulse. However, disturbing sculk shriekers enough can cause the hostile Warden boss mob to spawn, which is blind but capable of detecting entities through feeling vibrations in sculk, hearing, and smelling players when they are close enough.
Aside from this information, there isn't much that has been revealed about sculk shriekers. They don't appear to be flammable blocks, and can be stacked in one inventory slot to a max of 64 like many building blocks. Its hardness and blast resistance have also not yet been confirmed.
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However, as the timeline to The Wild Update shortens, Mojang will likely provide more information and maybe even more experimental builds of Minecraft for players to experiment with the blocks before they are released in earnest.