There are two things certain in monowhite: angels and taxes. - Serra, Planeswalker
This deck is built around tribal synergies and Avacyn synergies, for the most part. I didn't want any other angel apart from Avacyn as a commander for flavour reasons, and she works great with mass destruction tools such as Armageddon and Austere Command. However, this deck is also about the deficiencies in monowhite, rather than its advantages. The idea is to use the strengths of monowhite to disrupt our opponents until the disadvantages of the deck are outweighed by our board presence. I strived to achieve this by taxing my opponents until I have enough mana to basically obliterate the board and rebuild faster than any player.
Monowhite has two important disadvantages: ramp and card draw.
RAMP
Ramp is mostly achieved through artifacts like Marble Diamond and Worn Powerstone, and Caged Sun or Gauntlet of Power later in the game. Some cost reduction cards like Oketra's Monument and Pearl Medallion are also great in this section. Even though white doesn't have ramp per se, it does have ways to ensure having a land drop, like Land Tax and Endless Horizons. If you play Land Tax early in the game, I suggest using it always, even if you have to discard Plains. It helps a lot, as it makes you more likely to draw powerful stuff later on. Endless Horizons can be dangerous if you take out all the Plains in your deck, so I recommend taking out about 2/3 of the remaining Plains, just in case it gets blown up.
CARD DRAW
This is what you have to fight against: white is terrible at card draw. Most of it will come from ramp that is no longer needed, like Mind Stone and Hedron Archive. Mind Stone works great with Sun Titan, creating a great draw-train every time he attacks. Scroll Rack would be great if you have the card or enough money to buy it, but I don't use it, as I can't afford it. Cards like The Immortal Sun are surprisingly useful, as six mana won't be hard to get, and it helps with a little bit of ramp in the form of cost reduction and card draw. The same is true for Herald's Horn. Vanquisher's Banner is a great addition as well, it makes every angel more valuable as an ETB card draw spell and it buffs them up. Don't underestimate this last part, many times I happened to have Angel of Jubilation, Caged Sun and Vanquisher's Banner on the table, making Avacyn an 11/11 that kills in two hits!
Now that the disadvantages have been dealt with, let's look at the advantages.
TAXES
One ore two taxes in the early game will, almost always, turn the odds in your favour. Blind Obedience and Aura of Silence are your biggest assets, mostly because of their secondary abilities. The extort in Blind Obedience makes life gain easy, because you gain as much life as lost by all our opponents, and the targeted removal of Aura of Silence (that you can bring back with Sun Titan) works great in the mid to late game. Some of the taxation will come from angels like Chancellor of the Annex or, if you feel like it, Archangel of Tithes, even though I took it out as I felt it wasn't powerful enough. Magus of the Moat is quite weak, because it is a creature with no protection whatsoever unless Avacyn is in the battlefield, but it is too good when opponents don't have ways to deal with it. If you were able to get your hand on a Moat it would be, obviously, preferable.
MASS DESTRUCTION
It works perfectly with Avacyn and is also useful without it, as usual in Commander. I don't think it needs much more explanation, although I strongly suggest asking your friend group if they are OK with cards like Armageddon and mass land destruction. They are always replaceable by other mass destruction spells.
REANIMATION
Quoting Reya Dawnbringer, "You have not died until I consent". Graveyard recursion is great in this deck. Sun Titan is in itself great for the aforementioned synergies, but cards like Dawnbreak Reclaimer, Emeria Shepherd and Emeria, The Sky Ruin make your creatures impossible to get rid of. Twilight Shepherd helps against any kind of mass destruction, even your own, and Bruna, the Fading Light
or Karmic Guide complete the picture. I can't count how many times have I used Resurrection to bring back Emeria Shepherd, drop a Plains and bring back Karmic Guide, bringing back Baneslayer Angel. Stuff like that is always happening, so be very wary of graveyard hate, as you don't have ways to deal with it other than Mistveil Plains.
Let's talk about the rest of the cards and some other synergies.
Quicksilver Amulet works great with the most expensive stuff. It also creates a nice game-winning combo when people are short on targeted removal, having cast either Resolute Archangel or Felidar Sovereign the turn before, and when it comes back to you, dropping the other one right before upkeep.
Avacyn, Angel of Hope + Worldslayer
is nuts, but it has other uses. If you have either Endless Horizons or Land Tax in your hand, or any other mana rock, and Flagstones of Trokair on the battlefield, you may consider equipping the Worldslayer to any other creature. Your opponents will never see it coming, and you'll probably have the upperhand by ensuring your land drops and playing any mana rock. The same is true with any mass land destruction spell. Don't forget that Sun Titan can bring back lands from the graveyard, in case you blow up Emeria, The Sky Ruin or any other important land.
Apart from that, I added powerful things that are useful in many situations. Elesh Norn, Grand Cenobite is devastating, especially against tokens or goblins; Luminarch Ascension early in the game gives you a wide advantage, and Quarantine Field with double mana in the endgame can be game-winning.
POSSIBLE IMPROVEMENTS
Adarkar Valkyrie might be a nice improvement over weaker angels, like Linvala, the Preserver.
Rebuff the Wicked is being tested. I think it could be quite good, taking into account that once Avacyn hits the table it becomes the most important target, and usually the way to deal with it is through targeted exile removal. Other cards like Brave the Elements or Gods Willing may work, but they are specific to creatures, whereas Rebuff has the advantage of 1-Being unexpected, as who would expect a counter in monowhite, and 2-Being able to save any permanent you own of targeted removal.
Inheritance has been disappointing for me. It is only useful in corner cases where a mass creature destructor hits the board and you have plenty of mana unspent. Still, I might try to make it work, as I think the fact that I find it bad has to do with the fact that I play it as soon as I draw it, maybe I should wait until I can set up a boardwipe with lots of mana to cast it. However, it seems that Sensei's Divining Top would be a better choice anyway.
Iona, Shield of Emeria is very dangerous. In a format with cards like Bribery, keeping Iona in the deck entering a tournament would be crazy. For casual play though, is quite fun, as long as your play group is fine with it.
Rule of Law works very well in our deck and disrupts almost every combo strategy. The biggest downside is that it makes games incredibly boring, so I took it out. It is also quite a nuisance in the endgame, when you have lots of mana but you can only cast one spell.
Angel of Finality might be a good addition to fight graveyard strategies without disrupting our own graveyard, and it also triggers tribal effects.
Some more targeted removal like Disenchant or Oblation might be interesting. Oblation especially, as it can be used on your own stuff for a little bit of card draw.
More utility lands can be added, and they might work fine, but take into account that only basic lands work with some of the ramp cards like Gauntlet of Power or Extraplanar Lens. You have the option of going for a tech choice and change all the Plains for Snow-Covered Plains, negating extra mana to those who use normal Plains.
The Immortal Sun disables planeswalkers, which is nice since I don't use any, but I would love to use some. However, no monowhite planeswalker has more than one synergy with our deck. Any Elspeth is usually centered around tokens, and having a planeswalker that only synergizes with the deck in one ability seems a waste of space, at least to me. If you try her, feel free to try to change my mind.
THINGS TO KEEP IN MIND
Don't play mass land destruction with Avacyn on the board unless you are certain that they can't remove it at instant speed, or you have some way to protect her.
People forget about Emeria, The Sky Ruin way too easily, in my experience. Use that to your advantage.
Flip
Meld
