The spawn block is a block that can reset the spawn point of the player without having to sleep in a bed, And it can even work in the nether and end! When broken, the spawn block has a 50% chance of dropping just the bed, or the entire spawn block.
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Spawn blocks often appear in void cities, or NPC villages. At first, they spawn nothing, but if you use them right, they can spawn you, or anything else!
You can set your spawn point by right clicking on the block. This will set your spawn point to the location of the spawn block. If you can lead a mob within 2 blocks of the Spawn block, it will automatically spawn that mob every so often like a spawner. This is useful fornmaking mob grinders and such.
Sometimes, spawn blocks may appear as traps, spawning large amounts of dangerous mobs or bosses.