Dar'marets Temporal Twist

When you use your portent you are able to weave the weft of time and probability and capture the moment that you avoided and store it for later use. Instead of just replacing the roll with a portent you are able to 'swap' the dice results storing the original roll for later use. 

Exert: manipulating the temporal weft of the world to this extent is challenging enough to do at a minor level, storing these potentials is even more strenuous. When you cast this spell roll a D20, on a 10 or less the strain is to much and you are weakened. You lose hit dice equal to your Proficiency bonus, if you have no more hit dice you gain one level of exhaustion.