Harvest Festival

The harvest festival occurs when an individual has met the prerequisite conditions to evolve into a True Maou.

It is unsure exactly how many conditions are there, however one of them is to absorb the souls of 10,000 individuals.

It appears that within the Sorcery Dynasty Saurion, there exist a copy of a story book that describes events of individuals transforming into True Maou.

It should be noted that Orc Disaster was almost legible to transform into one as well (If not stopped by Rimuru), suggesting that the souls don't nesssesarily have to be human souls.