There are many fantastical worlds all separate from our own, with their own systems and creatures. Only powerful magics can transport one to another, some only triggering on death of the target, a few physically taking them away, and many giving a new body. Depending on the world, these feats may be learned by mortals or are exclusive to Gods.
But even Gods themselves cannot cross the rifts. They may enter other worlds within their bubble, but the bubble that is their world (often referred to as their universe or multiverse) is something they cannot traverse. If they could, one would have probably come to Earth and overtaken it. Instead, most Gods just pluck people out of it, living or dead, and either relocate or reincarnate them into their own world. Is it all the same Earth they keep taking humans from, or parallel ones? What happens when someone from those worlds actually is able to come to Earth? And what happens when those bubbled worlds have their rare moments of crossing over? Food for thought…
Whether these people become champions of their summoner, turn against the one who brought them here, attempt to live a life of peace, or tread their own path, a boon of some kind is usually bestowed. It depends on circumstances, and there have been tales of those brought to other worlds being given nothing at all or even getting cursed by their caller.
Many times, these worlds pull people into it because they may be… familiar with their systems. Because, for some reason, a lot of them share elements with games they may have played! That might be why most victims of these sorts of things are otaku. Those people know all about stat screens and leveling up and, most importantly, how someone can cheese the everloving hell out of them. It lets them essentially break the world thanks to having outside knowledge, inevitably exposing the flaws that come with a world's system being based around numbers and skills.
Maybe some places would be better off not calling in people from other worlds. How many times have the king or God or whoever else did the summoning have everything they worked towards ruined because they didn't like one or two of the people they called? Especially when the summoner casts the unwanted ones out on their own and they end up being the strongest ones? They definitely deserve it, though. Any world where someone in charge and with that power is being a picky-choosy jerk had it coming to begin with…
CRASH!
Aaaah, my everything! It aches! It… isn't? Wait, what?
"Pfffft! Oooohohohoho! Again, again!"
Who's saying that? I look over, and see some sort of woman. But what's more interesting is that floating sphere in front of her. It's showing… my room? And I'm there, at my desk, hyperfixating on a game. Wait, wasn't that what I was doing a few seconds ago-
CRASH!
… A car killed me. It got knocked off the road, flew into my room at the lowest level, and splattered me. No, it absolutely destroyed the house's foundation! Shit, my whole family was over for New Years! Don't tell me they also-
"Oooohohohoho! If only those vehicle-operating mortals were better drivers. Ah well…"
Right, that woman is… laughing. Laughing at my death…
"You… you think that's funny?!"
"Hmm? My, your soul got here fast. I thought I had a few more minutes to prepare a speech…"
A speech? Whatever the hell she's talking about, it doesn't matter right now.
"My family… Mom, Dad, Sis! What happened to them?!"
The bubble showing my death disappeared and I could see her face. Deep purple eyes with a golden shade looked at me with a mix of disappointment and uncaring, like I was asking for the time in a clock tower.
"Why should I know? So many reincarnation systems have been set up on Earth, and no one has a job to catalogue who goes where. They died like you, and they're being reincarnated like you."
… Fuck. I was going to make that new year count. I was going to get a job, a stable income, I'd finally be able to stop mooching off my parents! They'd be able to see me become successful and independent! A-and my friends, they're all online, they don't even know what happened to me! I spent so much of my time with them, and I wanted to do more! I want to live, I-
"Anywho, you can leave all the weight of your old life behind, because you've been isekai'd here!"
Isekai. I know that term. It means I've been brought to another world. One that's snatched me up after my death. There's so much shit on my mind, and this… this bitch is telling me to forget about it?!
"Now, I am a benevolent goddess…"
She stood up, her massive, golden, twintail hair bouncing as she did.
"And you deserve a cheat to make this new life easier…"
Her white and oddly skimpy outfit rustled while she got closer.
"Think of it like a gift from yours tru-"
And… she stopped in her tracks, then took a step back and sniffed the air.
"Y-you… you reek!"
… Come the fuck on. I've heard that enough in life, you're telling me that I have to hear it in death too?! Isn't there more important stuff going on here, anyway?!
"I have half a mind to just dump you somewhere with the strongest demons and just let you die for good! In fact, maybe I should…"
… No. No way am I letting this bitch laugh at my death and then kill me a second time!
"I've decided on my boon."
Putting on an act of being calm and collected with all these emotionally shattering thoughts and feelings is hellish. It's only because I've done it so much in my first life that I'm able to do it here. Even thinking about my time before now as a "first life" hurts…
"Oooohohohoho, very well! I can grant you any cheat, after all. Would you like the power of a hero? To bring over something from your world? I could even bring your family, all powered up from their reincarnated lives, so you can be reunited and happy!"
Wait, that last one doesn't sound too bad. I should take a deep breath, think over my options, and-
"My power… will be your role!"
-Aaaand I've already spoken my first idea. Damn it, me, why do I make the choices I make?! My "still composed on the outside"-self watches the woman pinching her nose and with her eyes shut (she's still not over my smell?) as she snaps her fingers.
… Then her eyes opened up and widened. As if she had just realized what she granted. Guess we both have "not realizing what we've just done until it's too late" as a shared flaw.
"N-no, wait, cheat NOT granted! Pick something else now, before-!"
A golden energy started to flow out of the woman, causing her great pain. Oh hey, it's coming towards me, this must be her power- AAAAAAH! MY EVERYTHING, AGAIN! IT HURTS EVEN MORE THAN DYING! I THINK WE'RE BOTH SCREAMING IN PAIN AT THE SAME TIME, I CAN'T TELL! IN FACT, IT HURTS SO MUCH I THINK I'M GONNA PASS OUT ANY MOMENT-"
Thud! Thud!