Here's a new level 9 chronurgy spell I just developed:

Diverge Future

Any feedback you could give would be well appreciated. 

Edit: Fixed the link problem

 "Page Not Found".  At least for me.

Ditto

Sorry, fixed

I like the idea of it, but it seems like it would start to suck RPing out the same day twice. Especially for the other players who would have to keep doing the same things over because “their characters don’t know, and it’s what they would do because they did it last time” like all the other characters in Groundhog Day.

Would it be better to avoid metagaming by making it affect your party members as well?

I think so, at least then they’re all in it together.

Gonna have to veto this one, Levi.

Look at it from the DM's perspective.  You've been leading up to this climactic BBEG encounter for the entire campaign.  You've carefully set the pieces, teased the hints, and are finally ready to unveil the huge twist you've been preparing to drop on your players for over a year, maybe even over two or three.  This is it.  This is the big payoff moment for the entire campaign, the thing your PCs have spent the last forever getting to level 17+ to do.

...and then they cast Diverge Future.  And get a free dry-run rehearsal.  They get to encounter your big, campaign-crowning moment for fakesies first, figure out your huge, world-shaking twist, and then go back to the morning before armed with the knowledge that Something Is Forever Different.  And even if they can't stop it with that foreknowledge, the moment is nevertheless pretty thoroughly ruined.

Yeah, the material component cost is obscene.  Yeah, even getting to cast this once in a campaign is something of an achievement in and of itself.  But the very nature of the spell, and the moments where players know they should use it, leads to the worst kind of plot-wrecking save scumming.

The general idea of 'you know what's about to happen' is encompassed in the Foresight spell, which is itself a heavily underrated  ninth-level spell that gives the character who possesses it advantage on everything and everything else disadvantage against them, because they get insight from just a flash into the future.  Diverge Future here is too much.  I would not allow it at my table, were I the DM.

Yeah, that’s a fantastic point. 

I agree. It's not meant to be available to the wizards as a default, and is mainly meant to be used as a plot device by the DM, and it should be very situational. The reason I included the super expensive material component is that it's not meant to be possible to use multiple times. It is a spell that I would not allow unless a very powerful NPC allowed them to use it in one circumstance. 

I might add a bit at the end saying that once you cast the spell, you can never cast it again or be under its effects again.

I realize that it would be extremely frustrating if there's a major plot point that the DM hasn't introduced yet, and the players just cast this and ignore all consequences. It is also annoying to allow the players to do the same thing twice. It's not meant to be used as a "Let's test this on the BBEG to see their weaknesses" but more of a "If we did this, what would happen."

I might even allow it that if the players got the results that they wanted, that they could make this timeline be the real one, so essentially they skip the second time, and just make the result they ended up with be the real timeline, just for the sake of not going through the same encounter twice. 

I guess that it does have some major problems that should be worked out, I was just wondering if the idea of it was appeasing. 

If your intent is for it to be a DM tool and a plot spell more than a regular thing, you may want to avoid making it available to players at all.  Have them locate a scroll with the spell on it, and have it be complex and arcane enough that even the most powerful wizard couldn't inscribe it from said scroll.  or perhaps they find the clock instead, and the clock can cast the spell - consuming itself in the process.  Some way of allowing the party to use the spell in the way they're meant to without letting yourself in for a whuppin'.

If you want something that could give the same results, but be less "involved" in the RPing, how about something like this? All spell statistics stay the same, but the effect changes as such:

Immediately when you stop casting the spell, time freezes in the world around you, and you create a divergent timeline that you are able to live through. Events play out as they would if you hadn't cast this spell, but you are in a temporary world. You are able to live through this world, discovering what would happen if you took a certain set of actions.

When you cast this spell, you gain a pool of [10-20] D20s. For the duration of this spell, you may choose to spend one die from this pool to force a creature to reroll any D20 roll he just made. You may continue to replace any D20 roll made until you have run out of D20s in your pool. 

This spell immediately ends if the result of a D20 causes you to die or become unconscious, or until you use an action to end the spell.

When this spell ends, either from reducing your pool of D20s to zero, becoming unconscious, or by ending this spell as an action, the strain of maintaining multiple realities pushes you beyond your limit and you immediately take 3 levels of exhaustion that can only be restored by multiple long rests. 

Yeah this runs afoul of what makes time magic so difficult to implement. The real constraint is the burden it places on gameplay. Regressing in time has historically been a miserable exercise in record-keeping on the part of both DMs and players.

I've considered spells that reflect time magic without record-keeping. One that I favor, but haven't yet tuned, allows a character to simulate a round twice and take whichever they prefer. The goal would be to allow the player to explore multiple options in combat, social encounters, and exploration.

Another option you could take would be to treat it as a very advanced version of plane shift that allows you to make big jumps in time. That would allow the DM to alter the setting without having to deal with the minutiae of a specific encounter.

I guess the main problem is this:

Say the players do this before the final boss fight. They go in, discover the BBEG's evil plot, and kill him.

The spell ends and they get sent back to their own timeline.

They try the same thing, but their rolls are different. This time they knew what was going to happen, but were unlucky and they died, for real. They tried doing the same exact thing, but the dice rolls changed, so they lost.

I guess that they should have the choice of staying in the new timeline, making that the real one. 

I might change this into a magic item like Yurei suggested. That might make it easier to control. 

 A one use magic item for a specific circumstance and perhaps a very short duration would be as far as I would want to take this in a game. Cool idea for a novel though.