This deck was made by Conley Woods who went 3-1 in U.S nationals.
This deck is different from many mono white decks, which tend to emphasize control or aggro. Conley’s list can play both roles effectively.
Ajani’s Pridemate is usually very big because most of the decks in Standard play a lot of creatures to trigger the soul sisters. Also, Pridemate can get big quickly. For example, turn one Soul Warden, turn two Pridemate, and then a couple more creatures from you and your opponent could turn the Pridemate into a 6/6 or 7/7 creature by turn three.
The life gain from the sisters also helps you beat creature decks because you gain so much life that it’s hard for your opponent to kill you. Also, Serra Ascendant becomes a 6/6 pretty easily against those decks if you have one or two sisters on the battlefield. For example, against Mythic, Conley went turn one Soul Warden, turn two Soul Warden and Kabira Crossroads, and then turn three he cast 2 Ascendants that were both 6/6 (his opponent had played two creatures).
You have 12 one-drops, which can all trigger Pridemate on turn two.
Kor Firewalker is solid because many decks play red. Blue/White and Mythic are really the only popular decks that don’t play red. Some people run Lone Missionary instead, but Firewalker is better because he doesn’t die to Cunning Sparkmage. Also, he lives through Destructive Force and Pyroclasm and saves sideboard space. You should board him out in a lot of matchups, but in game one he’s solid.
Survival Cache is good because you are usually able to draw two and gain four life for just three mana. Plus, it fits the mana curve well, since you don’t have a lot of three drops, just 4 Cache and 2 Oblivion Ring. Like Firewalker, you board it out a lot, but it is still solid main deck. It’s one of your best spells against UW Control and it triggers Pridemage twice.
Some people play less than four Brave the Elements, but that is a mistake. You should rarely sideboard them out. Conley was able to beat a counterspell and Path to Exile versus UW Control because he had three Brave the Elements that allowed him to attack for the win.
Elspeth is great because she can trigger the sisters repeatedly and provide chump blockers. Also, if you need to go aggro or destroy a planeswalker, she can give a creature flying and +3/+3.
The Tectonic Edges are good against manlands. Plus, they slow down ramp decks.
To sideboard against UW Control, you turn into a different deck. Your plan is to get a Luminarch Ascension active and protect it. You bring in 4 Ascension, 3 Path to Exile, 1 War Priest of Thune, 2 Oblivion Ring, and 1 Sun Titan. The Paths deal with Sun Titan, Baneslayer Angel, and Celestial Colonnade. The Oblivion Rings can deal with creatures too but they also fight other Rings that target your Ascension. War Priest of Thune stops Oblivion Ring and Journey to Nowhere. Sun Titan brings back countered spells or Tectonic Edge. To make space for the sideboard cards, cut 5 sisters, 1 Ranger of Eos, 4 Firewalker, and 1 Brave the Elements.
For your opening hand against UW Control in post-sideboard games, you want an Ascension or a big Pridemate. Elspeth is your other way to victory.
Linvala is there against Mythic and Cunning Sparkmage. Also, Path, Oblivion Ring, and Celestial Purge can deal with Sparkmage. However, don’t bring in Purge unless your opponent has a lot of red cards other than Sparkmage.
When playing the deck, you have to concentrate and pay attention because there are a lot of triggers to keep track of.