Having first selected the goal of the combo, I will attempt to reverse engineer toward a suitable method.
The goal in question being: letting my opponent(s) lose by providing them with 10 or more poison counters. Yes, this combo is a tribute to the most annoying “creature type” in magic to lose to: poison critters!
Insert anecdotal reference to killing an opponent with 10 swings of a swamp mosquito. Sidenote: I provided the sound effects myself!
Alas, the combo I will try to describe to the communal annoyance of us all, does not utilize the awesome power of the Swamp Mosquito. Instead, I selected it’s bigger brother (in the poison family), the marsh viper. Note that this serpent does not have to attack and come through unblocked or deal combat damage to a player. This creature has far subtler uses: Marsh Viper can ping (for 2 poison counters and 1 damage, but lets not focus on the latter).
Now lets find a way to abuse this notion, say via infinite poison counters?
Naturally, Marsh Viper (MV) has to have the ‘ping’ ability first, so this is where Viridian Longbow enters the mix. When equipped with the Bow MV pings for 2 counters per turn ("How the hell does a creature without any arms shoot a bow?" you ask. "Where’s the flavour?" you ask. Allow me to retort with either "This is a magical bow," or "The Bow is far easier to tutor," or "They merge into a poison spitting viper creature thing.") More damage must be dealt! Untapping MV seems to be a logical step to achieve more damage and subsequently more poison counters. Untapping MV repeatedly seems an even better notion on the road to infinite poison.
Luckily the Kamigawa-plane provided us with the enchantment Aura of Dominion (AoD). This enables us for 1 mana to tap an untapped creature we control to untap enchanted creature. Combined with the combo-happy Horseshoe Crab (HC) we now have the following combo:
tap MV equipped with VL to poison your opponent, then spend 1 colourless mana to tap HC to untap MV via AoD and then spend 1 blue mana to untap HC. Now we are back in the starting position with untapped HC and MV and 1U was used to achieve this. In other words we now have a poison-spitting apparatus for 1U for 2 counters a time.
Already a ‘potent’ combo…..but it has its limitations, namely mana. Thus we need to expand our mana pool... To infinity and beyond!
Let us first find a way to enable our lands to produce more of the needed mana. Land enchantments such as Overgrowth and Elvish guidance spring to mind, since both produce extra green mana. The HC however feeds on blue mana. The green mana therefore must be transduced to blue. Everyone’s second favorite manafixing snake fills this gap: enter the Orochi Leafcaller...
When we enchant an Island with both ‘growth’ cards it should give us UGGG when at least 1 elf is in play (say: ledgewalker? It could serve as an additonal/alternate win condition). Add a fancifull random karoo (lets say Gruul, Why?) and for instance you could have URGGGG (<-- thats why...URGGGG!). And when I SNAP my fingers you will (wake up?) untap two lands. How about the disjointed Island and karoo?
Only how will we be able to do this repeatedly? An Eternal Witness would suffice. So lets run through this: we tap the disjointed Island and karoo and add URGGGG to our mana pool, then we play Snap for 1U and we have GGGG left, three of those go into Eternal Witness to fetch the Snap back, netting us G and 2 untapped lands. Lather, rinse and repeat and go infinite. Half the green mana is transduced by the leafcaller to blue mana for the crab, the other half feeds the Aura of Dominion and we have perpetual poison spittage.
Only the first loop with the Witness and Snap could be problematic, since in subsequent loops the Snap targets the Witness (and vice versa). The potential problem is as follows: a bad play. Consider for a moment a situation in which the graveyard is emtpy.
Playing a Witness then is folly (another anecdotal reference, this time about folly: this one time at a prerelease tournament I was attacked with a Birds of Paradise that had Gathered Courage in my opponents second turn).
Luckily there is a creature in magic that can be discarded at instant speed (can you guess where this is going?):
Circling Vultures... Never again will you have to play a Witness without recurring something to your hand! This card is actually needed for the combo, but is so obscure it should be in each deck in existence (or not...).
Add a playset of Ravnica’s blue transmute and control cards such as Dizzy Spell, Drift of Phantasms and Muddle the Mixture, and you will be able to look for all the combo pieces except Marsh Viper. A pair of Trinket Mages are also a nice way to look for both artifact lands (Seat of the Synod and Tree of Tales) and the Longbow, so we add a couple of these to the deck. Lets take a look at what we’ve got now:
Poisonic Death ! Incomplete Decklist (non-Combo Pieces in italics).
Drift of Phantasms
Aura of Dominion
Muddle the Mixture
Tree of Tales
Seat of the Synod
Realizing I now have enough words to compete (seriously) I dont intend to launch into a description of how the deck should be played (I estimate this is such a bad deck it could actually enter a Bad Deck Match), for I also realize that that is pretty obvious. Additionally, I have already taken much pleasure in thinking up such a laborious combo and writing about it. So this is the point were my words come to an abrupt halt.