Editing Opposite Day

''9th-level transmutation'' 

Casting Time: 1 action

Range: 1 mile

Components: V, S, M (a diamond-dust-laced hourglass worth at least 25,000 gold pieces, which is consumed)

Duration: 24 hours, or until dispelled

Opposite Day is one of the mightiest spells a mortal creature can cast. You radically invert the fundamental laws of reality in a 1-mile radius centered on yourself for the duration.  

When you cast this spell, any attack roll, ability check, or saving throw made by any creature inside the range has its result inverted. If any of these types of rolls would have previously been successful, they now become failures, and if it would have been a failure, it is now a success. Additionally, any roll that would have advantage instead has disadvantage, and any roll that would have had disadvantage now has advantage. 

The unstable, precarious, reality-warping nature of this spell gives it a chance to completely break when you cast it. As soon as you cast this spell, flip a coin. If the result is tails, you lose the ability to dispel ''Opposite Day'' and you subsequently can never cast it ever again.