This article was originally written for the "Type Fun" blog operated by CPA members Oversoul and Al0ysiusHWWW. All such articles archived here were written by one or both of those individuals.
Probably my all time favorite casual combo deck is ProsBloom. You don't see it very often, but when you do, it gets attention--not necessarily because it's that great. It is an easy combo to understand the basics of, but it exercises tons of ways to go off. But I'm getting ahead of myself here. The combo itself is very simple. Let's see if you can figure it out.
That's right, 2(remove x cards) + U = 4x. Repeat. Looks great on paper. But there are a few finer points to the whole mess that make it unstable, slow it down, greatly benefit your opponent(s), or just otherwise confuse you. For the most part, running anything else but Prosperity means sacrificing the number of cards you draw.
The largest issue to the combo is that Cadaverous Bloom costs 3BG. It's a black card, so Dark Ritual does make things a little easier, but even that makes it a third turn drop without any other acceleration. Optimally, you'll be wanting to go off in less than four turns, with tutors making things a little tight. You could add a slew of artifact mana or mana-producing creatures, but that's adding extra "unreliable" cards to the fray. Additionally, Dark Ritual doesn't need to be pitched to bloom to get 2 mana net gain out of it. Which doesn't mean much, unless you're casting Prosperity looking for bloom.
Luckily, there was another card released with bloom that is so borderline broken, I'm surprised it's not in anything else. In some emergency circumstances, this card can be used to get the combo going until you can grab your Bloom. Squandered Resources can easily be paired up with any land search spells you can throw at it to not only ensure extra mana regularly, but mana of any color. At the time the deck was big, people generally used Natural Balance, which not only does the job, but does it very, very well.
Another issue is failing to run into another Prosperity after your first Prosperity. It's a scary feeling not knowing for sure what you're drawing into. And while running other blue X-draw spells could work, they generally cost extra mana, meaning the difference between 6 cards and 8.
Again, Luckily, you're playing black which means Tutors. While you could just run Demonic, every single mana is precious and your life doesn't really need to be 20 to go off. Vampiric Tutor does a great job of ensuring that your next draw will be a Prosperity. Or it can find a combo component that you so desperately need.
Sometimes, not even tutors will be enough. What good is a Prosperity if you only have one other card in your hand to remove with Bloom? Well, if it's either of these two, you're in no trouble at all.
Both Meditate and Infernal Contract exploit something that all combo decks should realize. If you're starting to go, you're unlikely to be able to recover if your combo fizzles out. Why not go all out? Skip turns, pay life, dump your library into the graveyard, do whatever you can do to win THIS TURN. Four cards might not seem like a lot, but it can be the difference between whether you run into a Prosperity or not, or more typically in this deck, three mana for eight mana.
Furthermore, you will need to drop something to play that blue mana, and while running artifact mana could help, I'd rather just do something that has synergy with Squandered Resources like Fastbond. It puts the final word on mana when you're finally going off and combines with Resources to say "Do what Bloom does just only with lands and you take a damage every time and with mana of any color."
Without putting up a model decklist, I want to discuss the practical value of this deck. It's a one-track combo deck that uses several methods to go off. After Fluctuator, it's probably one of the best decks for n00bs to combo to play. You get to manage the delicate balance between hand-size and mana, choose whether to keep a kill card or pitch it in hopes of drawing another before you fizzle out, or regret not playing a certain card a turn earlier since it would have made you combo second turn.
The last issue to discuss here is exploiting by-products of the combo for the kill. The traditional method is to use the slew of colored mana you can produce at the very end of your combo. It can be easily black mana, and therefore is perfect for Drain life. You need at least eleven cards (other than drain life) in your hand to do it and it's all in one shot. If you mess up, you lose all your mana, hand, library, turns, life, lands--everything but Cadaverous Bloom and maybe Fastbond with Squandered Resources.
Storm Seeker is, in most circumstances, better than Drain Life in this deck. It's an instant, so you don't have to kill them on your turn (I have no idea why you wouldn't), it doesn't require X mana, or even colored mana for the most part, it wont make you dump your hand or kill your lands, so if it gets countered, just cast another one (hopefully you'll be through your library anyway). And on that note, with two Storm Seekers, your opponent only needs to have 10 cards in hand for you to kill him/ her. Also, Storm Seeker makes multiple kills viable for multiplayer, or if an opponent has a large life total, you can just hit him/her twice.
Finally, if you're trying to modify the combo to include another draw engine besides Prosperity (Yawgmoth's Bargain or Stroke of Genius would be what I'd suggest), or just want to run another kill condition, Tendrils of Agony is an obvious choice. After a few Meditates, Contracts, and Prosperities, not to mention all the late drop enchantments, you'll easily have 10 spells played that turn.
ProsBloom will always be fun. Prone to disruption, it's very delicate, but effective. It proves that even refined combo decks can be casual.