Arena 6

Arena 6 is a special case since you’ll find the valkyrie Gondul here. Unlike the other 5 arenas you have conquered to reach this, Arena 6 hosts Surtr’s Hidden Trials and will feature the “Impossible” trial Difficulty. Here are the important details of the Hidden Trials:

• To activate the trial, you’ll have to obtain 3 keys by complete the Impossible trials in the other 5 arenas. If you cleared them before, you only need to return to a previous arena and activate the giant sword again.
• If you already have 3 keys in your possession, you can't get any more keys even if you complete the other trials
• After using the keys in Surtr's Hidden Trials, you can repeat these Impossible trials to collect the keys again
• The hidden trial's objective is random, which seems to be one of the other trial's objectives.
• There are three ranks: Bronze, Silver, and Gold. If you fail or achieved one of the ranks, the challenge will be over and you'll have to collect keys again to take the hidden trial
• Your reward will be based on your performance. The best rewards are acquired by completing the hidden trial with a Gold rank.

Here are the Key Trials objectives:

Trial I (Impossible): Defeat all enemies without getting damaged.

Trial II (Impossible): Defeat all enemies before timer runs out

Trial III: (Impossible) - Kill enemies while they're inside the gold rings.

Trial IV (Impossible): Enemies quickly regenerate health

Trial V: (Impossible) Kill enemies to increase time

One of the Hidden Trial objectives