Trial chambers are the copper-and-tuff combat dungeons added in 1.21, full of trial spawners and vaults that need trial keys. The map locates them so you can head to one with keys and an enchanted bow rather than finding it by accident.

• Ominous trial keys come from ominous trials — the harder variant — and open the ominous vaults with the better loot.
• Trial chambers generate in the deepslate layer, so look for them well below sea level near your marked coordinates.