Strategy

Hey there. I am happy to introduce a standard decklist for a new combo in MtG: Foundations centered around the cards:

The former yields absurd card advantage which is balanced by its fourth option, forcing immediate concession. This is where Harmless Offering comes in. After three turns, we may give control of Demonic Pact, killing the opponent in their upkeep. 

The rest of the deck is comprised of various methods for tutoring up the combo pieces, some removal and big finishers for grindy matchups. 

Insatiable Avarice gives us a straightforward manner to find any missing card and is often played on Turn 3 for card draw. Likewise, Beseech the Mirror can cheat Demonic Pact into play by sacrificing an enchantment or a possible Treasure Token.

Regarding the enchantments: Disturbing Mirth provides great card advantage, can sacrifice DP in the absence of HO and itself often sacrifices Gready Freeboooter. Once we Bargain it to another card, such as Beseech the Mirror on Turn 4, or Disturbing Mirth itself, we get a free 2/2. It can be heartbreaking to manifest a HO. The deck barely runs creatures and Artist's Talent allows us to quickly get through the deck. Upgrading it to Level 2 is great, and Level 3 synergises with Torch the Tower and the damage option of DP.

Right, concerning the removal: Torch the Tower can get rid of annoying Llanowar Elves, Heartfire Hero or (Insert anoying low health creature here). Importantly, we can sacrifice DP to Torch the Tower should we not have HO ready. Go for the Throat is great for knocking out big creatures as instant speed removal. Pyroclasm is a cheap removal against wide boards, though it's been a bit underwhelming in playtesting. On the other hand, Blasphemous Edict is a fantastic board clear, and Liliana, Dreadhorde General provides great removal, whilst defending herself and yielding more card advantage.

Getting the removal right is a bit tricky and depends on the meta. May be subject to change. 

Rite of the Dragoncaller provides a plan B should we lose either all Harmless Offering in the deck or Demonic Pact through discard, mulligan or, say, Deadly Cover-Up. There's an additional copy in the sideboard for grinding matchups, should either get removed, and the enchantment can be tutored up by any of the aforementioned cards. 

The landbase is straightforward and has mostly black lands; hitting these black manaresources is crucial for casting Turn 3 Insatiable Avarice. One can substitute the newly reprinted Scry lands; often we want to Scry instead of Surveilling to prevent either of the combo pieces of landing in the graveyard.

Sideboard

There's a lot more specific removal in the Sideboard. Against Enchantment centered decks we have Withering Torment; Bitter Triumph and Shealdred's Edict are used against Planeswalkers.

Untimely Malfunction often proves to be a great tool against blue control decks. The card may be used as a counter-counterspell to resolve our win conditions; alternatively we can redirect incoming enchantment removal, and Get Lost is a perfect target for this card. The artifact removal is also useful.

In grindy matchups or against reanimation decks, The Cruelty of Gix may steal their creatures and tutor up any of the desired cards. 

There's also an additional copy of Harmless Offering should the need arise.