@Rikmach  
Watch 4 seconds of this

With the Planeswalker Redirection rule removal, some of this text is going to become extraneous.

@Background Pony #2E66  
I agree she should be rakdos. I think the text is fine, except maybe we should change ‘each other… player’ to ‘each other.. opponent’ instead. While making her deal damage to her controller is flavorful, it kinda makes her unplayable IMO.

@IsBestPony  
You still have to live long enough to actually play Daybreaker, and be far enough ahead that she won’t kill you in the process. Reanimation and other cheats are a concern, I admit, but the same can be said of any big, dumb Timmy card.

a 5/5 flying indestructible with that effect is way too much man. Drop the indestructible or make her a 2/3. The cost is fine, if you keep that last effect. As it stand the card is way too powerful, and everyone would run it. It would be Slash Panther meta all over again.  
Another effect suggestion, that works well, would be “If Daybreaker the Unfettered deals combat damage to a player, prevent that damage and it deals 5 damage to each creature that player controls instead.” which would probably make her stay the same and reduce her cost from CMC 8 to CMC 6.

@FanOfMostEverything  
True. I guess I just want that bit of parallelism. Although now I’m thinking about how a cycle of alicorns might be done, given that we’ve got the right number. Way beyond my design capabilities though.

@Background Pony #2E66  
It is right there in her name. In Daybreaker’s case, she feels less like something pulled from the depths of the imagination of a mad oneiromancer than simply the sun in all its terrible glory. Thus, Avatar. Elemental might have worked too, but it didn’t feel like enough.

@FanOfMostEverything  
But Nightmare Moon also existed and you gave her the subtype.

@Background Pony #2E66  
The card works off of the premise of Daybreaker actually existing.

Wait, why isn’t she a Nightmare?

@dsagent  
It’d probably be unprintable otherwise. Don’t worry, just give her a nice ‘necklace’.

SO you can use it to kill yoursel?

@Background Pony #2E66  
Pillow Barrage: Good catch! Typo fixed.
 
Clash of the Heavens: If I cut the cost any more, it would cost the same as Wrath of God. I think it’s fine where it is.
 
Soul Swap: It’s actually based on a monoblack card. Mindslaver effects have been in black’s part of the color pie since Worst Fears. It would certainly work in blue, but I guess they’re going more for “gross violation of anything resembling decency or dignity” rather than “subtle mental manipulation.”
 
Coronal Ejection: This one’s interesting. See, “redirect” doesn’t appear to be a recognized term in Magic rules language. (It’s like English, but a lot stricter on structure.) Harm’s Way gives a good example of this. Basically, I’m not sure how to correctly phrase “The damage can’t be redirected.” And given how rarely Wizards prints such effects, I doubt I’ll find out any time soon.
 
Thanks for the comments!

Damn. That’s a nasty card, in the best possible way. Interesting to note that it feels both mono-red and like Daybreaker, when she herself feels Black-Red.
 
Regarding the other cards:
 
Pillow Fight: It should say “dealt by creatures,” rather than use the singular.
 
Clash of the Heavens: Much harder to make maximum use of than Extinguish All Hope given the nature of legendary creatures. Maybe the cost could be dropped a little more given that? Maybe.
 
Twilight’s Reassurance: That effect, that cheap, with Retrace? That is pretty nasty late game, with a little luck.
 
Foul Mooncakes: I love the modularity of it.
 
Endless Debate: I just love this one. Flavorful and fun.
 
Soul Swap: Great fun for multiplayer games, can create tons of havoc and potentially even take out two opponents. I’m not sure about it being mono-black though. Probably blue or black-blue.
 
Coronal Ejection: I’d add “or redirected” to the end.

@Background Pony #3CD3  
Yup. A creature with 0 toughness dies automatically. After all, damage equal to its toughness has been dealt to it.
 
And yeah, white and blue are your best bets for dealing with Daybreaker… though ironically enough, you can kill her with fire. It just has to be the right kind of fire.

So basically counter it or even better Counterbore it before it gets into play. If that fails then Oblivion Ring it or detain it or otherwise make it non usable. Infect still technically kills indestructible things right?