Nigun was the captain of the Sunlight Scripture of the Slane Theocracy. He was ordered to assassinate the Re-Estize Kingdom’s strongest warrior, Gazef Stronoff. Nigun was a deeply religious person who believed in the power he was granted through his position in the Sunlight Scripture. This made him arrogant, but once he was beaten, he tried to throw away his own subordinates and save his own life.

Being the captain of the Sunlight Scripture, Nigun had been involved in many illegal activities. Recently he was sent to eradicate a demi-human village but was stopped by Blue Rose, whose leader managed to scar Nigun's face. He kept the scar as a reminder of his defeat.

After being taken prisoner by Ainz, Nigun and the Sunlight Scripture are being questioned, tortured, and experimented on by Neuronist Painkill under Demiurge's supervision. Nigun was the first one to die, tortured to death while being questioned.
(Source: Overlord Wikia)