The Minecraft 1.21 update will bring a brand new structure called a trial chamber that will have several new trial spawner blocks. These blocks will spawn hostile mobs based on the number of players approaching them. It will also have a cooldown period, during which it will offer gamers rewards for defeating hostile mobs. However, these new spawners will summon various kinds of mobs.
There is a particular method of knowing which mob a trial spawner could summon.
Note: Since the Minecraft 1.21 update is still in development, the types of mobs mentioned below are not final and could change or increase. Mojang may even add newer mobs in the future that can be summoned from the new trial spawners.
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Types of mobs that can be summoned from trial spawners in Minecraft 1.21 update
Breeze

If a trial spawner is surrounded by new chiseled tuff blocks, it will spawn the new breeze mob. These are wind-related hostile mobs that will shoot wind charge projectiles towards players. These wind charges can knock them back and also interact with certain blocks in the world.
Zombie

Those trial spawners that have mossy cobblestone around them will summon regular zombies in the trial chambers. These hostile mobs are, of course, some of the most basic and will be relatively easy to kill.
Husk

Husks are another common hostile mob that will spawn from trial spawners, particularly chiseled sandstone blocks around them. These entities can inflict hunger effects with their attacks as well.
Slime

Slimes are another hostile mob that can spawn in the trial chamber structure from new spawner blocks. Trial spawners surrounded by moss blocks will be able to summon the blocky, hostile entity.
Silverfish

Silverfish are one of the most annoying hostile mobs since they are small and can move quickly. They are also extremely hard to hit. Trial spawners that will have stone bricks around them will spawn silverfish in trial chambers.
Baby Zombie

Baby zombies are also extremely annoying, hostile mobs that can spawn inside the new trial chamber structure. Trial spawner blocks that have cobblestone and mossy cobblestone around them will summon these small entities in the game.
Spider

Regular spiders will spawn from trial spawners that are surrounded by stone blocks and cobwebs. Spiders are also one of the most common hostile mobs that generate in almost every dark area.
Cave Spider

Cave spiders, on the other hand, will spawn from trial spawners that are surrounded by stone blocks and cobwebs, as well as podzol blocks and red mushrooms. These are rarer spiders that mainly spawn from regular spawner blocks found in mineshafts. They can inflict poisonous effects with their attacks.
Stray

Strays are rare variants of skeletons that mainly spawn in cold biomes. These can shoot special arrows with slowness effect from their bows. If a trial spawner has packed ice surrounding it, it will summon stray skeletons.
Skeleton

Finally, those trial spawner blocks that are surrounded by bone blocks will spawn regular skeleton mobs. They are fairly common in the world and can be easily killed by a seasoned player.
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