Addiction
Nial didn't really want to work on multiple Darkness Pillars at the same time but his subconscious was clearly telling him that he was capable of doing it.
In the worst case, he could also store the excessive dark energy inside his Heart of Darkness. However, that was actually not something he wanted to do, which Nial realized pretty early.
Thus, Nial did something extremely stupid, something nobody except Nial would do…He started to work on all of his remaining Darkness Pillars at the same time! It was something he chose to do out of a fit, just because he felt like doing it.
This increased the danger of making a mistake more than fivefold as Nial had to divide his focus on each of the remaining Darkness Pillars that had yet to be created. At the same time, it increased the time each individual Darkness Pillar took to be created, which was only obvious because Nial had to divide his focus.