The Christmas Trials is a gamemode in The Mimic, which was a part of the Christmas Event 2021. Completion of this gamemode on Nightmare Mode will reward the player with the badge "Christmas Trials Nightmare Completed," a limited-time lantern which the player can find in their inventory.
In the event's story, Krampus is attempting to take over Christmas so nobody can have Christmas, leading to the player(s) having to stop him.