Reincarnation Stories
Reincarnation is a common trope closely entwined with isekai. I’d estimate about ⅔ of isekai stories are reincarnation stories.
Simply put, in a reincarnation story, the main character dies in the prologue or first chapter. Then their soul (or the author’s equivalent) is put into something new. Unlike in other types of isekai, returning “home” is almost always a known impossibility for reincarnation stories.
Usually the main characters retain all their memories and skills from their previous life, but this isn't a constant.
There are three main types of reincarnation: humanoid, monster, and object. And then there are three other main types of reincarnation: reincarnating as a baby, replacing a dead person, and just kinda appearing out of nowhere.
Here’s a handy chart with examples:
To go into more detail:
Humanoids might not be human, but they have the same basic limb structure. They’re usually placed in a city or group of people, and the “what the hell is going on” is strong. Generally, reincarnated souls aren't something new to the people around them, but it being good or very bad is a coin toss.
Monsters aren’t always horrible beasts. They might be cats or dogs, or epic dragons. But you will find a decent amount of nasty creatures that the locals want to kill.
And objects are things that are typically non-sentient. Trees are very popular. Rocks and bricks are kinda popular for the lolz. But I’ve also seen hats and vending machines, so have at it.
Reincarnating as a baby is… controversial. Mostly because some authors who take this route obviously have a breastfeeding fetish. Not all, and yes it’s an easy way to have a character know nothing of the world without anyone thinking it odd, but… yeah… babies gotta eat. Also this route usually has the main character be basically helpless for a few chapters as they age. Main characters being helpless and having no agency isn’t popular with GameLit readers.
Replacing a dead person is often accompanied with amnesia or someone pretending to have amnesia. Generally the dead person doesn’t have friends or family who notice the sudden personality change, but sometimes they do. This is an easy way for the author to make the main character an adult (or teen) without the handwaving of…
Just kinda appearing out of nowhere. Poofing into existence with a new adult body. When done with humanoids, it’s more common to be done through a portal or summon, not reincarnation. Main character opens a book on Earth, and suddenly finds themselves on Dirt, no dying needed. But if they legit die on Earth, it’s more common to wake up as a full-grown monster or object. Sometimes a god-type thing gives them an introduction and choice of what they want to be. With humanoids it’s almost a requirement that they be given an exposition dump by a god-type thing before getting to their new home. Those stories also tend to rely more heavily on game elements than the others.
So tldr: reincarnation stories are stories where someone dies and wakes up somewhere else and/or as something else.