A trial spawner is a type of spawner found only within trial chambers. It spawns mobs in waves, with the amount depending on the number of nearby players in Survival or Adventure mode. When all of its spawned mobs are defeated, it ejects a similarly weighted quantity of useful consumable items or trial keys, and then goes on cooldown for a period of 30 minutes. For more infos : https://minecraft.wiki/w/Trial_Spawner