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 Sorcerer's Dynasty Sarion 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.

Known Harvest Festival

 * Harvel Festival of Tempest. Rimuru evolved and awakened into a True Demon Lord through using the 10,000 souls of the Subjugation/Invasion Army. Revival of the Tempest citizens killed by the Blood Shadows along with Shion, Evolution of the Onis into Fair Onis and Dark/Evil Oni, Strengthening of the every monsters in Tempest under Rimuru's rule.
 * Evolution of Rimurus subordinates after his victory against the Invading army of the Eastern Empire. Ocurred after the evolution of Zegion, Gabil, Ranga and Diablo. Zegions Harvest Festival only strenghtened Apito, while Diablo held his Festival back as much as possible. Gabil and Ranga Festival strengthed their followers but no evolution amng them did occur.
 * During Dinos attack in the Labyrinth, Benimaru completed all conditions for his evolution. His Festival caused Souei to evolve into a Middle tier Divine Monster Spirit.