That Time I Got Reincarnated as a Slime (Tensei Shitara Suraimu Datta Ken), also known as TenSura, is a Japanese light novel series that spawned several manga adaptations and two anime seasons. The TenSura manga was especially successful, becoming the first manga based on a light novel to reach 20 million total copies in print. In fact, the TenSura franchise was so successful that almost every manga in the wildly popular isekai genre (in which one or more persons are transported to another world in where they have to survive) is based on its blueprint, creating a serious problem of lack of originality in the industry.
Light novels are Japanese short (usually 50,000 words) novels targeted at young adult readers, commonly illustrated in a manga-inspired style. They are relatively cheap and can be bought at the same price as a manga volume (6 US$). Light novels have become an increasingly important part of the Japanese publishing industry, with sales climbing higher and higher each year. One of the most popular light novels of 2021, was actually Demon Slayer (adapted from the popular manga). While TenSura didn't quite reach the same heights as that, it was still massively popular, and much of that popularity came from its fresh premise.
That Time I Got Reincarnated as a Slime tells the story of Satoru Mikami, a Tokyo corporate worker living a monotonous life until he is stabbed to death by an unknown assailant. While dying, Satoru hears a mysterious voice and wakes up to discover that he has been reincarnated as a Slime in a fantasy world. His feelings at the moment of death influenced his new species and the skills he acquires, particularly the ability called "Predator," which allows him to devour anything and mimic its appearance and skills. After meeting and freeing the Storm Dragon Veldora, the slime receives his new name Rimuru Tempest and eventually becomes the leader of all beings inhabiting the Great Forest of Jura, founding the nation of Tempest. Rimuru's strength and memories of the "real world" help his nation to quickly grow in strength, drawing the attention of other powers, from monarchs and heroes to the creatures known as "demon lords", whose ranks Rimuru eventually joins.
The difference between TenSura and other popular isekai manga is that the protagonist does not arrive in the new world in his own body, nor in a human body, but starts as the monster that is usually recognized as the weakest one in RPG-style games In these games, slimes are the enemies that the player "farms" in the early stages to gain experience, but Rimuru does the opposite. After he wakes up in a vast, underground cavern complex, he spends months eating and absorbing nutrients, materials, and other monsters, emerging as a powerful creature, whose weak and cute appearance is only a ruse. This reversal of the classic RPG tropes was a unique spin on the isekai genre and particularly on its forefather, Sword Art Online, whose plot is based on the protagonists entering a massive virtual reality game.
The reincarnation genre gives mangaka and writers the chance to explore completely absurd premises, and That Time I Got Reincarnated as a Slime doest that straight from the title. However, the growing popularity of the genre, energized by the success of the TenSura franchise, has made it stagnant very quickly. It is also true that some recent examples of isekai are actually satires and parodies that are helping the isekai genre get a much-needed breath of fresh air. Still, it's undeniable that reincarnation has become a hugely popular sub-genre of isekai. While it probably wasn't the first, That Time I got Reincarnated as a Slime definitely helped popularize reincarnation isekai, leading to a glut of pale imitations.