The deck in a few words: use Seer to put powerful Undead on top of your spellbook to discard with Cemetery Rats or Shallow Grave, then use Raise Dead or Carrionette to summon them directly into your opponent's face.

Explanation of my card choices:

• Seer is probably the best Avatar for this strategy, as we need both spells and mana to function and they are the only avatar that can draw ~1.5 cards per turn (if you count scrying 1 as half of a card), they can set-up our mill cards, make sure we hit tricky threshold costs by drawing the perfect combinations of sites, while still allowing us to draw a spell and tapping to draw a site in the late-game.
• Cemetery Rats, Shallow Grave and Forgotten Tomb serve as the main engine of the deck, sending a powerful minion to the cemetery for us to reanimate. It is important to understand that the cards do not hurt us in any way, yes, even if you mill two powerful magics you are not losing anything, MtG players have already learned that milling is not card disadvantage 20 years ago, feel free to learn the same lesson today.
• Clairvoyant, Funeral Pyre and I am Colossus! are here to banish unwanted minions from both cemeteries, so that a future Raise Dead can get us a good card no matter how unlucky we are.
  Clairvoyant may look redundant, given our Avatar, but we can use her to scry our spellbook and if the card we see is bad, we can use our Avatar to see a second card, while if the card is good, we can use our Avatar to scry our atlas.
  [[I am Colossus!] also counts as a great target for Raise Dead in the late-game, if you get lucky, or manage to clean up the cemeteries.
• Kythera Mechanism (and Highland Princess) is here to allow us to select the card we want out of Raise Dead.
• Lightning Bolt and Thunderstorm are here as great Wind-Elemental removal, it is not hard to stir them in a way thet they always damage the target we want, but since we are playing Kythera Mechanism they get a lot better.
• Sisters of Avalon are here to fix our mistakes, in case we draw an expensive minion too early (or if we manage to draw an Infernal Legion at any point in time) we can rely on them to bail us out, draw us a card and prepare our cemetery for reanimation later.
• Skeleton Mage is here to extract extra value from our magics, they can play our removal, Landmass or even take advantage of our opponent's cemetery if we match elements, it also works great in combination with Cemetery Rats and Shallow Grave, since we cannot control what our second card is going to be, sometimes we will mill an Ordinary magic.
• The Hexham Haunts is a slow and clunky card, but the ability to play them from the cemetery means that we virtally get to draw a card every time they die, their main job is to block, protecting the Avatar's site, then maybe surface when the coast is clear for some chip damage, or to trade with an utility minion.
  It is also a great answer to Root Spider.
• Daperyll Vampire, Renatus Trueblood, Army of the Dead, Draconian Bonekite, Infernal Legion and Nightmare are our money cards, trow them in the cemetery, reanimate them for 3 mana directly in your opponent's face, and the game is either won on the spot, or close to done.
  A special note on Infernal Legion, no, we cannot cast them, yes, that does make the card worse, no I do not care, it is still the best target for Carrionette
• Second Wind is a cheeky movement card, we can play it when our opponent leasts expects it to let one of our unit step twice and strike on the opposite side of the board, or keep it in our back pocket in our cemetery untill it will do the most damage, is also a great card in combination with Cemetery Rats and Shallow Grave, since it lets them, virtually, draw a surprise card sometimes.
• Bury is self-explanatory, it kills 90% of all our opposition, it is also best saved for a powerful enemy minion or artifact, since our minions can win any fight they get into.
• Carrionette and Raise Dead are the key cards of our deck, as they allow us to get a powerful minion from any cemetery and also lets us summon them anywhere in the realm.
• Grapple Shot, Whirling Blades and Earthquake are the best magics in our elements, there is no reason not to play them in our deck.
• Landmass is a great card in a deck filled with powerful and expensive spells, it might be surprising to hear, but sometimes we even get to cast the powerful and expensive spells.
• Book of the Dead is basically an Omphalos for us, a more expensive, more fragile and worse in all the ways it matters Omphalos, but an artifact that draws us an extra card each turn nonetheless.
• Toolbox is the best card in the game, it is a two mana, 10-choiches, modal spell, which leads us perfectly to...

My Collection:

• Zap! the best DD finisher in the game, no fuss, 1 mana, you are dead.
• Disenchant and Dispell can kill all the most annoying cards in the game, Ring of Morrigan? Jihad? Chains of Prometheus? Gone.
  We are playing both of them just in case we need them in the early game while we don't have the threshold to play one or the other, also because annoying cards may come in pairs.
• Willing Tribute is a great and cheap way to get our opponent from 10 life to DD in one turn.
• Border Militia is our answer to an aggro deck that over-extended with their sites, it can gain us upwards of 15 life by just blocking over and over.
• Arc Lightning is our answer to a cheeky 4 power Voidwalker and/or aggressive minion trying to get all up in our buiseness.
• Monks of Kobalsa is a generic midrange minion, just in case we draw an extra Toolbox and have nothing better to do.
• For Bury, Grapple Shot and Landmass please see above.