Well, set up the common event checking if your player is pressing the button that dashes. By default it's shift, so set up a common event like:
Conditional Branch: If Player Pressing [shift]
>MP -5
>Wait frame(s): 30
>Else:
>MP -2
>Wait frame(s): 30
So basically, if the player is pressing shift, the player will lose 5 MP every 1/2 second where as if the player is not holding shift, they will only lose 2 MP every 1/2 second.
I'm a little confused as to how a player would gain back their fatigue in your request... since your player would constantly be losing MP in all cases.
Now if you want the screen to get kind of blurry would be pretty difficult if you want it to be animated...but what you can do is set up another Common Event that checks the levels of your player's MP and pull off a semi blurry effect. It wouldn't blur, but you can pull off some sort of effect by tinting the screen a lot in different combos. Like this:
Label: START
Conditional Branch: If Player [Ralph] MP == 0
>Loop:
>Label: TINT
>>Tint Screen: 0, 0, 0, @20 Frames (wait)
>Conditional Branch: If Player [Ralph] MP >= 15
>Break Loop
>Jump to Label: START
>Else:
>Tint Screen: -15, -15, -15, @10 Frames (wait)
>Tint Screen: -10, -10, -10, @30 Frames (wait)
>Tint Screen: -25, -25, -25, @20 Frames (wait)
>Tint Screen: -15, -15, -15, @40 Frames (wait)
>Jump to Label: TINT
>End
>Loop Above
Else:
>
End
The code above
should work. What it does is constantly check if your player has 0 MP. If they do, then the screen will tint to normal brightness. Then it checks if they have less than 15 MP, if they do have less than 15 MP, the screen will begin to wildly tint at different shades of dark. Each time it runs through the tints, it'll jump back to tinting the screen to normal again where it'll again check if the player still has less than 15 MP. If at some point the player has 15 more more MP, the loop will break and jump to the very beginning to check if the player has 0 MP. (They won't since they will have 15+ MP at that moment). Anyways, it just keeps looping and checking. Basically, what happens is that the screen will tint crazy while your player has less than 15 MP only if regenerating. So, you can have 5 MP but the screen wont start tinting until you reach 0, when you reach 0 your screen will tint until your reach 15. When you get to 15 the screen tints will stop until you reach 0 again.