WARNING: The following contains spoilers for Season 2 of That Time I Got Reincarnated as a Slime.
The first half of the latest season of That Time I Got Reincarnated as a Slime saw the series protagonist, Rimuru Tempest, get a significant boost in power. After his people are attacked, Rimuru takes it upon himself to awaken as a True Demon Lord, ascending to the upper echelon of the world he reincarnated into. But what exactly is a "True" Demon Lord, and what does it take to become one?
Following a trip to visit his students in the Kingdom of Engrassia, Rimuru was attacked during his return to Tempest. He was sealed inside a barrier, restricting many of his abilities before being confronted by Hinata Sakaguchi, a Holy Knight of the Western Holy Church. She revealed her intent to destroy Tempest, which was supported by a report by Seoui that the city was under attack.
After a narrow escape from Hinata’s onslaught, Rimuru returned to Tempest. As he arrived, he discovered many of his subordinates and citizens had fallen to an attack by the Kingdom of Falmuth. After being shown the body of his close aide and friend, Shion, Rimuru began to fall into despair. Even Great Sage, Rimuru’s Unique Skill, was unable to resurrect the victims of the siege.
However, as Rimuru is about to place the bodies of the fallen in his stomach’s subspace, Eren and the rest of Kaval’s Party return. Before Rimuru consumes the bodies, Eren stops him, telling him of a fairy tale only known by a few in the Sorcerous Dynasty of Thalion. The story details Milim’s ascension to Demon Lord after destroying a nation in retaliation for her pet dragon’s death. As a result, the dragon resurrected, albeit without a soul. At first, Rimuru is skeptical that the citizens might be soulless monsters upon their revival. Still, upon the mention of the barrier surrounding Tempest, which was meant to weaken the monsters inside, Great Sage indicates there's a 3.14-percent chance that the souls are contained inside.
Following this revelation, Rimuru asks Great Sage about becoming a True Demon Lord. Great Sage informed Rimuru that he had already acquired the Demon Lord Seed and needed to provide it with nutrients. According to Great Sage, the Demon Lord Seed is an indicator that an individual has the necessary magicule volume, skills and other things required to ascend to Demon Lord Status. Rimuru had acquired his seed after defeating the Orc Disaster.
To nourish the seed Rimuru required 10,000 human souls, with the 20,000-strong army of the Kindom of Falmuth providing the perfect source, as they'd begun to march on Tempest. This would begin the process known as the Harvest Ceremony, in which Rimuru would begin his transformation. Then, in an uncharacteristically cold fashion, Rimuru faced the army himself, slaughtering all 20,000 soldiers in a few moments.
This began Rimuru’s transformation. During the evolution, both of Rimuru’s Unique Skills, Predator and Great Sage evolved into Ultimate Skills, while Rimuru himself developed into a Demon Slime, increasing his body mass and magicule volume exponentially. In addition to the vast increase in his strength, all of the subordinates named by Rimuru were granted gifts in the form of Unique Skills.
After waking from his transformation, Rimuru was greeted by the revived citizens of Tempest, including Shion. Rimuru saw such a drastic increase in power that he was able to unseal Veldora, something that he never thought he'd be able to do so soon. Rimuru is not the only Demon Lord featured in this season, however.
After celebrating the citizens’ resurrection, the leaders of Tempest begin to focus on the perpetrator of the attack on their nation, who is revealed to be Demon Lord Clayman. It is revealed that, despite his title, Clayman was planning on using the souls of the Falmuth army to help himself ascend to True Demon Lord status. Despite having claimed the title of Demon, Lord Clayman had not completed the Harvest Ceremony.
This brings about an interesting dichotomy within the ranks of the Demon Lords, as it seems that not all those who claim to be a Demon Lord in title may not be one in truth. In the Reincarnated as a Slime anime, only Milim and Rimuru have been confirmed to be True Demon Lords. As this comes with a significant increase in power, those who have not ascended to Demon Lord must have their own ways of cultivating power. Those who have attained the status of True Demon Lord, meanwhile, appear to wield nearly divine levels of power.