MAIN COMBOS:
Tree of Perdition + Triskaidekaphobia = Win on upkeep
Tree of Perdition + Catapult Fodder (Flipped) = Opponents life total set to 13, and a 20 damage fling.
Assault Formation + Behind the Scenes = Most creatures in the deck being able to swing for high unblockable numbers.
Tree of Perdition / Necrotic Ooze + Cryptolith Fragment / Triskaidekaphobia + Snakeform = Opponent set to 1 and pinged by Cryptolith Fragment or Triskaidekaphobia on upkeep
Necrotic Ooze (with a Tree in the yard
Tree of Perdition + Soul Seperator + Triskaidekaphobia / Cryptolith Fragment = Opponents life total set to 1, ping for 1.
Triskaidekaphobia + Tainted Remedy = Opponent ping 1 and you gain 1 every upkeep.
Tree of Perdition + Tainted Remedy = Tap once to lower life to 13 (or lower if blocking that turn), tap again to make Opponent gain 7 which lowers them to 6, tap again to deal 7.
Journey to Eternity + Catapult Fodder (Flipped) / Skullport Merchant = Creature recursion for 5 mana, and the ability to double up on a fling or card draw (Journey itself can also be used to protect Tree of Perdition until a combo piece can be drawn or tutored).
DECK HISTORY:
Version 1: Abzan Doran list with Tree-Trisk Combo in it
Version 2: Sultai list similar to this one but very expensive due to making a tri-color land base work without being too slow to survive aggro decks despite being full of high toughness blockers.
Version 3: This (probably final) list runs just Golgari to make the land base work cheaper and more consistently. This is a combo list with a treasure package in it to give it a better curve from turns 1 to 3, as prior lists leaned heavily on just playing tap lands till three drops and tutors.
SIDEBOARD DETAILS:
Treasure package can be sideboarded out to run fog effects and removal for more aggressive or tempo oriented matchups and hand disruption / combo protection for control decks.
CARD LEGALITY AND INTENDED FORMAT:
This deck would be entirely modern legal if Tangle out of the sideboard would be replaced either with Fog or maybe Clinging Mists. Deck is also two cards (Necrotic Ooze & Snakeform) away from being Pioneer legal as well. Sorin the Mirthless would take those slots perfectly.
Casual was selected as the format only to indicate the intended level of play for the deck; as it's certainly not going to be well tuned against a tournament meta.
Should it be improved to compete with highly competitive (and far more expensive) decks, dig up would be replaced by demonic tutor, the land base would include fetches and shocks, and likely the treasure package would just be replaced with main deck removal and hand disruption, maybe some recursive method of fogging such as spore frog and tortured existence or something similar.
CARDS THAT WANT TO BE IN HERE:
Sorin the Mirthless wants to be a two of in here, haven't figured out what to sub out without breaking up certain combos (Behind the Scenes & Assault Formation could probably make the cut and make the deck closer to legal in a fresher format).
Dockside Chef is basically Skullport Merchant but two mana cheaper, no treasure on ETB, BUT can sacrifice any artifact instead of just a treasure. Better suited to Gilded Goose.
Birds of Paradise would also love to make an appearance, though as a three of, and likely replacing Concealing Curtains or Cryptolith Fragment + two other cards. Sylvan Caryatid is more important to ramp to 4 on turn 3 as most of the combos in the deck start at 4. Albeit turn one bird turn two Sylvan and Goose is a PHENOMENAL start and puts turn three at 6 mana.
If Birds come in, (and Skullport is replaced with Dockside Chef) land-count can probably go down to 20.
At the end of the day the deck could easily fit these cards when removing Necrotic Ooze, Snakeform, Cryptolith Fragment, Assault Formation, and Behind the Scenes. Although more playtesting would be necessary to determine how well it runs that way compared to this way.
FINAL THOUGHTS:
Shambling Ghast was replaced with Gilded Goose, I still think that despite the non-synergy with Skullport Merchant that it's a better turn one into turn two Deadly Dispute, functions as a turn on for Catapult Fodder, and basically makes food into treasure tokens anyway.
Cryptolith Fragment might need to be a third copy of Assault Formation as that card really turns on the pressure in this deck and the mana base wouldn't even have to change a bit. Although, as a one of it gives us something to tutor up that is one mana cheaper (and requires no specific colors) for the snakeform win and it lets you deal the one damage on the same turn you combo vs. Triskaidekaphobia doing it on your next upkeep.
Also circumstantially should it flip, this deck loves a 1/4 with flying and death touch that deals 3 every time you turn it sideways at someone so, I haven't quite lost faith in it yet.
Plus it's a mana rock you can curve into with Gilded Goose to untap on turn 3 with 4 mana to create a food token and Deadly Dispute it, whereas Assault Formation does nothing in multiples.
Definitely a hard call, but with Dig Up I think having several one of helps more than it hurts in an incredibly multi faceted combo deck.