The sculk Shrieker blocks were first spotted in Minecraft LIVE's global reveal stream of The Wild Update. They are one of many new, sculk-based blocks that can be found in the deep dark biome with its subterranean towns.
The Sculk Shriekers can be found in Minecraft: Bedrock Edition's creative inventory (allegedly when experimental gameplay has been toggled on or while you are playing the 1.18 development build). They are currently not fully implemented but will be available when The Wild Update goes online in 2022.
There's still enough information from the deep dark biome and sculk to draw some conclusions. Mojang will likely provide more details as the update nears.
When Minecraft is fully implemented, sculk shriekers can be found in the subterranean dark biome. You can break them with any tool, but farming hoes are the best. They have a mixture of black and cyan colors, but also feature bone-white parts.
Sculk shriekers, like many other sculk block, are sensitive to vibrations, sounds, and entities can send them through sculkgrowth and into other blocks.
Sculk shriekers can be disturbed in Minecraft by vibrations, sound or redstone signals. They emit a loud “shrieking” noise that creates a dark pulsing atmosphere.
For approximately 30 seconds, the light will dim in and out before returning to normal. The possibility of mobs spawning in the dark pulse is unknown. The hostile Warden boss mob can spawn if enough sculk-shriekers are disturbed. It is blind, but it is capable of sensing vibrations in the sculk and hearing players when they are near enough.
This information is all we know about sculk-shriekers. These blocks don't seem to be flammable and can be stored in one inventory slot up to 64 times, just like other building blocks. The hardness and blast resistance of the block are not yet confirmed.
Mojang will likely offer more information and possibly even more experimental Minecraft builds to players as The Wild Update gets closer.