It’s my understanding that as the DM you are able to do anything you want?

Within reason. Your role as DM is to balance fun and challenge equally. As for forcing things upon players without reason, it’s not particularly against the “rules” as much as it is just plain rude. Say they were to pick up a cursed sword which when picked up, begins talking to the player, giving them a sort of bloodlust where they feel compelled to kill. That’s acceptable. However something such as the DM deciding all of the sudden “you now can’t hold this sword you’ve been using the entire game because you’re too strong” has faulty reasoning and is absurd albeit comedic.

Well I suppose you can kill and replace everyone but you have to have a good reason or it is rude.

Sorry I thought you were talking about the Republicans,my bad.

^^honestly, my first thought was that this was political as well

It’s an interesting idea, but I would strongly recommend getting player approval first. “But I want it to be a surprise!” you might be thinking. Sure, but you don’t have to tell them exactly what they are agreeing to. A DM who wants to explore certain themes doesn’t have to say “I’m planning on doing this, this, and this. Is that okay with everyone?” They can ask “Is everyone okay with exploring this kind of theme?” instead.

For example, you could say “How would everyone feel about a campaign where you get a taste of power at the beginning, but then you are all brought down to normal?” Or “I was thinking you all could play the children of some famous adventurers, but we might do a session where we play their higher level parents to get some ideas and plot hooks going.” Neither reveals your surprise, but both prepare them for the fact that they don’t get to keep their 10th level powers.

@SQUEEKY2 Coming to this Q was kind of like buying a lottery ticket. The thought process was fun until the reality hit!!! ;]

You can do whatever you want as DM, and there are many different views about what’s good and bad to do as DM.

I would say that the main issues I see with your plan are:

1) You’re forcing outcomes, and forcing them all to die, and not telling them that’s what you’re doing, at which point you are not refereeing a game situation. Instead, you’re telling them to make characters they want to play, and then deceiving them that they have a chance to survive, and then you plan to force to kill them no matter what they do.

2) If they figure out that you’re forcing outcomes and not letting them play in a dynamic game situation where what they do matters, then you’re setting up an expectation that you’ll keep doing that. To avoid that, I would either tell them up-front, or afterwards, and promise not to do it again. Or, admit that you intend to reserve the right to force a lot of outcomes and railroad what happens.

Personally, I would tell them you intend to run a short prequel campaign, and to make higher-level temporary characters for that. Then set up a difficult but not impossible-to-survive situation and run that fairly. Have it end though and establish the past situation whether they survive or not, but have effects on the later campaign situation based on what they manage to do. That way, the prequel campaign actually has stakes and effects on later play.

So they may likely all be wiped out, and the stage will be set for the long-term later campaign, but there will be chances for some or all of them to survive somehow and accomplish some things, which will have some effect on the starting situation for the later campaign.

At least, that’s what I’d tend to do with your idea.

@Zaku has a great answer.
Don’t take full 100% control of RPG games and allow no input.
RPG games are supposed to be a “Choose your own adventure” If someone wanted the creator to dictate everything I’m sure they’d play a regular video game/ another game in general.
I play RPGs but never been a DM so I’m not super sure:)

No. Some players have built a character for years. Just swap them with new characters.