The trial key is an item added in the Minecraft 1.21 update. It looks like a copper key with a dark blue spooky skull as the handle.
You can get the trial key from multiple sources inside the Trial Chambers and only inside this structure.
Here are the multiple sources from where you can get the trial key
You can get the trial key from a chest near the entrance with a finding chance of 6.8 %
You can also get the trial key from decorated pots in the corridor with a finding chance of 3.6 %
You can also get it as a reward for completing the trial spawner’s challenge with a finding chance of 50 %