The "Spiraling Doom" deck is an extremely reactive deck: most of your decisions will depend on what your opponent is doing and how your opponent is trying to win the game. The key card in your deck is Birthing Pod, which lets you sacrifice a creature to go get a slightly more expensive creature out of your library. Once you have Birthing Pod on the battlefield, use it to find the perfect creature for every situation! If you need to kill a creature, look for Skinrender. If you need to deal with a powerful artifact, enchantment, or land, find your Acidic Slime and take it out.
If your opponent's deck relies on attacking you right away with small creatures, stay back on defense. Creatures like Young Wolf, Strangleroot Geist, and Viridian Emissary make perfect blockers. All of them give you added benefits if they trade with your opponent's creature in combat. Viridian Emissary replaces itself with a basic land, helping you to cast your more expensive spells like Myr Battlesphere and Bloodgift Demon earlier, and the other two come back to the battlefield stronger thanks to their undying abilities.
If your opponent decides to sit back and wait, you can often force the issue. Attack with Strangleroot Geist or Young Wolf when your opponent has several 1- or 2-toughness creatures. Blocking them won't be an attractive option. After combat, cast a Mortarpod and equip it to a creature. Then if you need a blocker, sacrifice that creature and it will return to the battlefield ready to block. If you have Skinrender instead, you can just kill your opponent's biggest creature and scare off the rest with the 3/3 blocker.
Because of Birthing Pod, your main deck includes at least one creature for most situations you can expect to face, but your sideboard lets you focus even more. Against opponents playing for the long game, Despise and Distress can prevent them from casting their best spells. Gravedigger and Myr Sire are more creatures that are good against aggressive opponents. Dismember can also buy a lot of time.
Birthing Pod is a flexible card that empowers a variety of strategies, but for the most part, it's all about creatures. If you want to customize your deck, look for creatures with "enters the battlefield" triggers and "dies" triggers. The Dark Ascension card Geralf's Messenger has both. Or you could look for creatures that punish specific decks. If you're facing lots of decks with artifacts, Oxidda Scrapmelter can be a valuable part of your Birthing Pod toolbox. If you're expecting lots of creature removal, Sheoldred, Whispering One is a great way to bring your creatures back from the grave.
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