Seeing a deck port from Standard to become a competitive Modern deck is not all that surprising. The card pool isn’t so large that new cards don't stand a chance, and there are pushed cards in every new set.
But to see a deck make the leap from Standard to Legacy is not something you see every day.
The queen of prowess, however, is no surprise. A 1/2 haste creature certainly isn’t exciting, even for a single mana, but with cheap spells, Swiftspear regularly grows to 3/4 or 4/5. Legacy offers a bevy of cheap cantrips, so it won’t even cost you cards in hand to pump her
If Swiftspear is great, then is Stormchaser Mage playable? It turns out that the answer is yes. The extra mana is a real cost for a deck trying to get on the board as quickly as possible, and spending even your second turn playing a creature rather than digging through your deck can be a problem, but you need to have a creature in play to maximize the value of playing so many spells.
For a deck that's maxed out on spells, and filled with ways to manipulate the top of your deck while interacting with your opponent for free, Delver of Secrets sounds perfect. A flipped Delver ends the game quickly, and you can still play and attack with your Swiftspear on turn 2 (often triggering some prowess as well).
Vendilion Clique feels a little out of place in this deck to me, but it’s certainly a great spell. It can interact with your opponent, help dig deeper into your own deck, threaten planeswalkers, ambush creatures, and it attacks for a lot of damage. My first choice would be Snapcaster Mage, but Clique is solid.
Bedlam Reveler is so sweet. You’re looking to churn through your deck, casting spells that will let you dig deeper and power up your other creatures. If the opponent can interact and remove your board, Reveler will make sure you’re always ready with more gas.
Gitaxian Probe is at its best here. Not only do you get perfect information for your interaction, but you get free prowess. Ponder and Brainstorm let you dig through your deck quickly while also making sure your Delvers will flip as soon as possible.
Daze is the perfect interactive spell for a deck looking to get on the board and win quickly. Your opponents won’t have the time to build up their mana base to pay for Daze, so it acts as a hard counter more often than not. When it isn't, Force of Will can pitch in.
U/R Prowess wants to win in a hurry, and this suite of cheap burn spells should do the trick. Lightning Bolt is the best of the bunch at instant speed, but Chain Lightning to the face will serve a similar purpose.
Price of Progress ranges in value. Some decks can never ever beat it, and some practically ignore it. The ceiling is high enough that it’s exciting to have as an option. Thunderous Wrath is a terrible card to have in hand, but another great top deck. Brainstorm can even set it up when necessary.
U/R Prowess has little-to-no interest in playing a late game, and it’s one of the fastest and most efficient weapons in the format.