The Hell Trials spell is an incantation that is an indispensable part of closing the gates of Hell. The spell acts as a signal of the trial's completion.
The person who finishes one of the Trials of God, has to recite an incantation: Kah nuh ahm dahr. The phrase is most probably Enochian.
Unlike most spells seen on the show, this spell has certain side effects. Upon finishing the incantation, the reciter doubles over in pain and their arms glow a fiery orange for a few seconds. The glow subsides and the person regains his composure. After finishing two trials, the reciter's health takes a turn for the worse. When the last trial is completed (and the gates of Hell are closed), the person who underwent the trials would die.