Ever since when Professor Onyx was spoiled for Strixhaven: School of Mages, deck builders immediately thought to combo the card with an under-appreciated uncommon from Onslaught: Chain of Smog. Taking advantage of the buzz, Magic Untapped has put together an Oathbreaker build pairing the two cards together as the deck's primary win condition.
Despite the combo's high mana cost of eight (4BB for Onyx plus 1B for Chain), playtesting has shown that it can fire off as early as turn three.
Commander & Signature Spell (2):
As mentioned above, the primary win condition is getting Professor Onyx out and casting Chain of Smog, targeting yourself. This allows you (the target) to create a copy of Chain of Smog to, once again, target yourself with, repeat ad nauseam. Each time, Professor Onyx's triggered ability (essentially a word-for-word Syphon Soul effect), well, triggers. Game, set, and match.
Now, about how to get this eight mana, two card combo to fire off on turn three? It'll take a little bit of luck with the opening draw, but here's how it goes down:
Turn zero/one: Gemstone Caverns, Swamp, Charcoal Diamond or Helm of Awakening
Turn two: Swamp, K'rrick (using four mana or Helm of Awakening discount and spending six life to cast)
Turn three: Swamp, Professor Onyx (using four mana or Helm of Awakening discount and spending four life to cast), Chain of Smog (using one mana or Helm of Awakening discount and two life to cast).
Failing that, you'll get there soon enough.
As for what else is in the deck, well there's a fun assortment of Madness cards that you can pitch and (if you can afford it) cast off of those Chain of Smog discard effects. There's also a handful of kill spells to muddle your opponent's plans, as well as a minor extort theme as a way to nickle-and-dime your opponents as you build to your smoggy end. Finally, there's an old combo circa Urza's Saga that's included as a fun and effective secondary win condition.
The combo works off of Pestilence, which, for one black mana per trigger, deals one damage to all creatures and players. Having Urza's Armor on the board means that you won't be taking the damage even though everything else will. Unfortunately, if you kill off all of the creatures on the board, Pestilence, too, goes away. That's where Cemetery Gate, a wall from Homelands comes into play. It has protection from black and, thus, will never take Pestilence damage. This guarantees that you will have at least one creature on the board to keep Pestilence around to take everyone and everything else out of the picture. Get the picture?
We hope you have fun building and tweaking our Oathbreaker Professor Onyx build to suit your own liking and play style. The deck is a fun one. Well, maybe not so much for your opponents. Hey, you can't make everybody happy.