As a divination wizard, I love the Portent mechanic. There is little better than pointing at an ally about to unleash their biggest attack on the BBEG and telling them, 'you just rolled a 20' (or whatever number I have that's high enough for them to hit). It makes me feel like a proper divination wizard.
Unfortunately, I have found there are few combat spells that have the same feel of being able to directly influence others' fortune available. That is why I have tried to capture the feeling of Portent in spell form.
Alter Fate
2nd-level divination
Casting Time: 1 action
Range: Self
Components: V, S, M (a glass bead)
Duration: Concentration, up to 1 minuteBy focusing your inner eye on the near future, you are able to influence the weave of fate. When you cast this spell, you roll 1d20 and record the number. As a reaction, you can replace any attack roll, saving throw, or ability check made by you or a creature that you can see with this foretelling roll. You must choose to do so before the roll. This foretelling roll can only be used once.
At Higher Levels: When you cast this spell using a spell slot of 3rd level or higher, you may add or subtract 1 for each spell level above 2nd to the die roll, with a minimum result of 1 and a maximum result of 20.
I am having trouble finding a proper way to estimate this spell's power level. I realize that there is a reason divination wizards only receive their third Portent die at level 14. That is why I have tried to limit the spell by requiring concentration and an action to cast.
The part I am most worried about being unbalanced is the 'at higher levels' part. Does this make it too easy to get critical hits or misses?
Best Answer
As is, it is overpowered.
Replacing dice is a very strong feature, and the divination wizard signature. Making this a 2nd level spell means any wizard can use it... and many will be able to abuse it.
Divination wizards can abuse it, starting level 6.
As wizards of the Divination School recover spell slots when casting divination spells, a 6th level wizard can spam this 9 times each day - and still get plenty of level 1 slots to keep casting. It becomes even more silly at higher level ; at level 17, it means 46 uses per day, not even considering arcane recovery.
It gets worse at level 18, with Spell Mastery
Any wizard will be able to cast this at will. If you don't like the roll you just made, cast it again. You'll eventually roll high / low enough to auto-succeed any ability check, or ensure the guy you intend to charm to auto-fail.
Some solutions?
Option 1: don't homebrew a spell
Making this diviner unique feature a spell makes it available to anybody. It will feel less unique, and remove the spotlight from that subclass. You may prefer to homebrew a feat to allow more portent dice (1 or 2), or short rest recoveries (probably too much). It may be the good way to see it used more often.
Option 2: make it higher level
Bluemoon's 5th level is probably a good start - but depending on the class restrictions you have in mind, level 6 would be a good way to ensure no warlock ever gets to spam this on short rests, or diviners can't use recycled spell slots to cast it again. Following that path, I'd advise you to change the upcasting ability to add more dice, instead of changing their results. A +3/-3 probably does not justify the use of a ninth-level slot, but 4 dice might.
The problem with this option, as pointed out by @Ben Barden, is that it removes access to the spell to low-level casters. You should probably look for other options.
Option 3: 1-round duration
If you like the idea of wizards spamming this (I do!) - limiting its duration is probably the way to go. If a PC wants to spend the entire fight predicting rolls for his allies - let him spend all his actions doing it. If you choose this solution keeping it a 2nd level spell, you'll probably want to limit its use to combat rolls - barring ability checks, to avoid Spell Mastery out-of-combat shenanigans.