Day's Undoing at your end step = Strictly better cantrip. GG.
Incorporating Day's Undoing into Storm. Hopefully, this will become a Tier-1 deck.
How to Play
Hope you like solitaire!
Step 1: Get Leyline of Anticipation into play. This is crucial because it lets us cast Day's Undoing without ending the turn.
Step 2: Cantrip at their end step until you find a Day's Undoing. If we can turn on a Pyromancer Ascension during this time, that is ideal.
Step 3 (Optional but preferable): Gitaxian Probe to make sure you're all clear to combo off.
Step 4: Resolve Day's Undoing. Make sure you have mana to cast a ritual or have mana floating from said ritual. If you have an active Pyromancer Ascension, the copy resolves and the original remains on the stack (this is relevant because you can resolve the original after you're done doing your next round of shenanigans. In addition, the copy resolving means that future Day's Undoings won't shuffle it back in).
Step 5: Up your storm count. Use rituals to make more mana, and turn some blue with Manamorphose. Continue to cantrip. Once your hand is empty or only has lands and/or Day's Undoing(s), proceed to step 6.
Step 6: Repeat steps 4 and 5 any number of times until your storm count is at least your opponent's life total.
Step 7: Cast Grapeshot.
Step 8: Laugh maniacally.
Step 9: Win.
Sideboarding
Sideboard is currently in development.
Empty the Warrens: If your opponent has hexproof, you can win on your turn because you combo'd off on end step.
Epic Experiment: "I have 12 floating, what now? Oh that's right, search for Grapeshot and/or Day's Undoing.
Lightning Bolt and Dismember: These are here for if you have to play a fair game of Magic. They can kill things. 'Nuff said.
Mindbreak Trap: Can handle other combo decks, especially good in the mirror.