Willpower
Nial had 10 days left before he had to leave the darkness portal.
However, he wanted to finish the torture of completing the primary Trial as quickly as possible.
It might be an advantage that he had more time inside the darkness portal than outside, but that was not especially helpful if Nial was not allowed to have a longer break.
A long break would allow him to digest all the pieces of information he had received throughout the last 20 days, and make use of it to help him have a much higher combat prowess.
Unfortunately, Nial understood that he should be patient and that a break was something he could receive once he was done with the primary Trial.
Patience was something he had gained by enduring every single death, without trying to rush completing the Trial before he understood the crux behind the Darkness Doppelganger's abilities.