Harvest Festival

The harvest festival occurs when an individual has met the prerequisite conditions to evolve into a True Maou or can kill Rimura Tempest

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 storybook that describes events of individuals transforming into True Maou.

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

Known Harvest Festival

 * Harvest 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 every monster in Tempest under Rimuru's rule.
 * Evolution of Rimurus subordinates after his victory against the Invading army of the Eastern Empire. Occurred after the evolution of Zegion, Gabil, Ranga, and Diablo. Zegions Harvest Festival only strengthened Apito, while Diablo held his Festival back as much as possible. Gabil and Ranga Festival strengthed their followers but no evolution among 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.