I feel I need to provide the contrary viewpoint and say that this card, while usable, is really pretty disappointing. I want to like this card because it forms part of the major theme of mystics. There are spells like Shriveling that let Mystics use Will to be good at fighting, there are spells like Rite of Seeking that let Mystics use Will to be good at investigating, and this is ostensibly the spell that lets Mystics use Will to be good at evading. Unfortunately Mists is really weak compared to these other two. The problem is that Shrivelling is much better than a basic fight action (+1 damage), and Rite of Seeking is much better than a basic investigator action (+1 clue), but mist is not really better than a basic evade action. In theory it gives you an action compression bonus just like the other two (+1 move), but that bonus is far less useful because you rarely want to use it. Indeed, it is so much less useful that it is barely makes up for the penalty of sometimes having to discard a card.
The most important use of evade is to keep an elite monster distracted while the party fights it, but if you are doing this you do not want to move away from the monster. Evade can be good against regular monsters for many characters who are not fighters and find it much easier to evade and let other characters do the fighting. But if you are a Mystic you have the option of taking your fighting spell instead, and it is almost always better to do 2 damage to the monster rather than evade the monster. Evasion can be better against monsters with 3+ Health if you can just leave and ignore them, and the +1 move bonus of Mists seems primarily designed for that exact circumstance. But FFG seems really reluctant to put many such monsters in their adventures, it seems like the great majority of the monsters either have Hunter, or have doom on them, or are otherwise better to kill than leave alone. To be fair, there are some monsters you can run away from, and there are some monsters you are better off evading than fighting because you get a penalty if you kill the monster, and here is where Mystic evading shines. The problem is that even in these cases +1 move isn't a great bonus because often when you encounter the monster you are not done investigating or otherwise interacting with the location it is on, and you still don't want to move away. This is compounded by the fact that Mists is not an event, it is an asset, so while it is conditionally useful, you have to play it before you know whether that condition will be satisfied; once you have played it, you are likely to end up using it mostly at times when you don’t want to run away.
Despite all of this, mists would be fun to play around with if it didn't take up a slot. But the fact is that it takes up an arcane slot, a slot that is in very great demand for most Mystics. This means that even if you decide to have a bit of whimsy and put this in your deck, you often regret playing it rather than waiting for a better spell to put in that precious slot. Mists is best for characters who have high Will and low Dex who have access to Mystic cards but don’t really use their arcane slots and have the resources to fool around with conditionally useful cards. Not a very broad selection of characters, to say the least.
If you don't have many expansions, Mists is a better card because you do not have many other options, if the only way you can fight is with the two copies of Shrivelling in your deck you may want to put this card in just so you can do something when you don't have Shriveling. On the other hand, if you have lots of expansions, the real nail in the coffin for this card might be Sword Cane, which lets you use Will to evade without spending charges or using an arcane slot.
