Nial received a rather simple explanation of the three trials he had to pass.
The Nightmare trial would throw him into a Nightmare, forcing him to re-experience the worst time of his life. He would have to witness the worst time of his life once again even though he had already sealed the memories in the deepest parts of his subconsciousness.
Nial was pretty sure that he wouldn't witness his mother's death again. He already had nightmares of that in the past and he overcame them.
On the contrary, Nial was certain that his Nightmare would be related to his two lifetimes. Even if that was not the case, it would be somehow related to his being as Nial Lacardia. Nial could bet his life on that.
Other than the Nightmare Trial, the Trial of Truth and the Trial of Power seemed rather simple.