RPGMemes

This is DnD 5e yes.

The "divination wizard" in 5e has a power where at the start of the day, they roll two d20 dice (or at high levels, they roll three d20 dice), then they put them aside for later. At any point during the adventuring day, whenever anyone the wizard can see makes any d20 roll for any reason, the wizard's player can say "wait stop, don't roll the dice for that, instead your result is THIS dice" and they use up one of their stored dice. This feature is called "portent" - it represents the wizard seeing the future.

So for example, if you roll a high number (let's say, a 17) - and an enemy wizard casts a very nasty spell, causing your friend to have to make a saving throw, you can say "this looks scary, and if you fail this check, you'll die. I'm going to declare that you rolled a 17 for this save."

Most players when they see this feature think about the high numbers, those are "powerful" feeling because they let you succeed at doing something when it really matters - and this can make a big difference - when the outcome of a story comes down to "can we convince the important NPC we're the good guys?" one roll can make a huge difference, and getting to make the check knowing what the outcome can be is phenomenally powerful. As players you can even plan to make full use of this - coming up with a plan of action where everything hinges on a single die roll is normally very dangerous in DnD, but when you can force that roll to be good, it's an incredibly powerful tool.

One thing that many players dont consider, however, is the raw power of a very low roll. You can use these portent dice when enemies make checks, so if you have a small number, you can make an enemy fail. One of the most common uses of this will be to cast a spell that causes a monster to make a saving throw, or suffer serious consequences (e.g. a spell like "polymorph" can turn a monster into a snail for an hour, effectively letting you win the fight against them with a single roll.) - these are commonly referred to as "save or suck" spells, and the big weakness of them is, if the enemy makes the saving throw, nothing happens. You wasted your turn, and used a powerful spell slot and got nothing. These spells get significantly more potent when you have portents.

(Another incredibly powerful use of portent is to use dice in a pair - let's say you need to lie to the head of the city guard to cover your tracks, or you need to win a contest of strength against a minotaur. If you have both a high and a low die in your portent pool, you can make the contest, and give yourself the high die while giving your opponent the poor die. This lets your tiny puny wizard win a tug of war against a Minotaur, or lie to the guard captain and have him absolutely believe you - and you know in advance what the outcome will be!)

In general, the most powerful monsters in DnD 5e are called "legendary monsters" and they have a special power where - whenever they fail a saving throw, they can say "actually, this is really bad for me, I'm going to treat this like I passed the saving throw" - this means you can't just defeat the final boss with a single spell, they're protected against it. Generally these monsters get to do this three times, which means you need to hit them with a "save or suck" effect four times to win the fight, but monsters can have a smaller number, or a larger number of these, depending on their design, and what the dungeon master wants out of the fight.

In this case, our Dungeon Master wanted the final boss to last a long time, and be a big epic affair, so she gave the boss ten legendary saves. (He was a god, after all.) - so she was confident that our group would have to fight him the normal way, and she was right. There was no way for me to use the portent fight to cheaply defeat the final boss in a single spell...

However - initiative - the skill check you make at the start of fights, is a d20 roll as well... so you can use the portent dice to change that outcome. When a whole group of people is fighting one very dangerous boss in DnD, "initiative" becomes one of the most important rolls in the game. The monster gets relatively few turns, but they're very impactful. The difference between them rolling high and going early vs rolling low and going late is massive - using the portent die to make the final boss act last effectively served as a free turn for the entire party. This is just as powerful as many "save or suck" effects.

Our group had played through a full campaign of monsters, seeing my wizard use this one tool to mess with enemies for the entire time, so by the time we reached the end of the campaign, over a year after we started, they were all pretty used to seeing it have a big impact on the game - however, knowing what we were fighting, they were expecting it to be a fairly minimal impact there. High numbers would be helpful, but they'd be forcing one roll to be a success in a fight that needed ~50 successes to beat. The boss could use legendary saves on most spells, so the low numbers wouldn't be helpful at all. So everyone was surprised to see the lowest die roll have probably the greatest impact of any die roll in the entire campaign.