Mayhem Devil/
Zulaport Cutthroat - These two are grouped together because they do very similar things.
Mayhem Devil is obviously more powerful since it can hit any target, but if either one stays on the board it is what allows me to put my opponent on the defense.
Cauldron Familiar and Witch's Oven - This is a great combo by itself, let alone adding Mayhem Devil and/or Zulaport Cutthroat. This combo gives me a constant blocker, gain life, AND drain my opponent. Adding any of the other cards to this combo makes it that much more powerful.
Claim the Firstborn - On the subject of Witch's Oven this is a small removal package that's pretty fun. I can take any small creature, attack with it, the sacrifice it to give me food while forcing my opponent to lose their creature. Even without Witch's Oven, this can allow me to take a blocker out of the way to swing in.
Dreadhorde Butcher - This is probably my favorite card in the format. It may not be the best, but it is the most fun for me. A turn two Dreadhorde Butcher NEEDS to be answered or it will take over the game. What is great about it is that I can sacrifice it in case my opponent tries to exile or bounce him. This will ensure his damage pulls through.
Chandra, Acolyte of Flame - It is pretty obvious that she is here for her second 0 ability. These tokens have multiple purposes that are very useful in this deck. Attacking in each turn is not to be overestimated even if they are only 1/1s. Also, even if my opponent has blockers, they are sacrificed at the end step which still allows me to continually drain my opponent with Mayhem Devil and/or Zulaport Cutthroat. Her -2 allows for multiple uses for a majority of my instants and sorceries when needed.
Chandra, Torch of Defiance - This one may seem a little off synergy, but she is so powerful it has shown to be worthwhile. One fault I noticed with this deck was the lack of card draw. I would be out of a hand and if any combo piece was removed I was a sitting duck. I have since added Light Up the Stage as well, but her utility as shown to be useful.
Zariel, Archduke of Avernus - This card is in testing but I believe it to be promising. I wanted it for the additional fodder and the fact the he can not make my Cats and Elemental Tokens more damaging on attacking.
Fatal Push/Murderous Rider - Standard removal in the format. Murderous Rider's lifelink has also come in handy.
God-Eternal Bontu - A 5/6 with menace that recurs itself is powerful and a draw engine on top of that. Combos with Zulaport Cutthroat and the even more powerful Mayhem Devil.