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.
Casting Time: 1 action
Components: V, S, M (a glass bead)
Duration: Concentration, up to 1 minute
By 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?