Day:
When a creature within your revealing light makes an attack roll, you can use your reaction to force it to make a Constitution Saving throw against your druid Spell Save DC. On a failed Save, the creature is Blinded and has disadvantage on all Ability checks, attack rolls, and Saving throws made with Intelligence, Wisdom or Charisma until the end of its next turn.
**Blazing Light:** When a creature is blinded in this way, it is blinded for 1 minute. At the end of each of its turns, it can repeat the Saving throw, ending the effect on itself on a success.
Additionally, a creature blinded in this way takes an amount of radiant damage equal to your druid level at the start of each of its turns.
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Night:
When a creature makes an attack roll against you, you can use your reaction to force it to make a Constitution Saving throw against your druid Spell Save DC. On a failed Save, the creature is Blinded and has disadvantage on all Ability checks, attack rolls, and Saving throws made with Strength, Dexterity, or Constitution until the end of its next turn.
**Crippling Dark:** When a creature is blinded in this way, it is blinded for 1 minute. At the end of each of its turns, it can repeat the Saving throw, ending the effect on itself on a success.
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Cosmic Reversal
At 14th level, your control over day and night reaches it's pinnacle. As an action, you can reverse the time of day in a circular area with a diameter of up to 100 miles centered on you. In this area, the sky appears as it would in 12 hours, such as appearing to be midnight when it's really noon, and you gain the features of the time of day it appears to be.
This change lasts for 2 hours or until you end it as an action. Once you use this feature, you can’t use it again until you finish a long rest, unless you expend a spell slot of 5th-level or higher to use it again.
##### When does it change from day to night?
Ultimately, that is for you and your GM to decide, as no answer perfectly fits every player or campaign. The answer that makes the most sense in the world of the game is that it changes over at sunrise and sunset, or maybe at a set time every day, such as 8 AM and 8 PM.
Consider having a grey area in between day and night, maybe just a minute or two where you gain the features of both day and night, or maybe neither, or pick and choose which abilities you want from each.
Alternatively, if you don't want to track exact time, you can change it so that you remain in the same form no matter the time of day, and switch between the two whenever you finish a long rest.