Oh look, a potential turn 3 infinite in pioneer! That isn't to say that this deck will win on turn 3, but the shell is capable of it. The combo is simple: Blood Bairn + Kaya's Ghostform + Renegade Rallier
results in an infinitely large Blood Bairn by sacrificing the rallier with a ghostform attached, returning the rallier, then returning the ghostform with rallier's trigger, then repeating until you have an arbitrarily large Blood Bairn. Grusome menagerie makes this a 1 card combo if you already have revolt activated through a Fabled Passage or Renegade Map. Now, let's look at the turn 3 kill
Turn 1: play swamp, Stitcher's Supplier, milling 3
Turn 2: play a land, play Blood Bairn, hope it doesn't die.
Turn 3: play a land, sacrifice Stitcher's Supplier to Blood Bairn, milling 3. You need to mill Messenger's Speed and Kaya's Ghostform in your 6 mills. Play Renegade Rallier and begin the combo until you have a big enough Blood Bairn. Then, return Messenger's Speed, attaching to Blood Bairn,instead of ghostform with your rallier, and swing for 10,000 hasty damage.
Sure, it's very Magic Christmas land, but Gruesome Menagerie makes this a one card combo that your opponent has to respect once the cards are in the grave. Hateful Eidolon can be the 1 mana creature for infinite draw with the combo, and you can use Key to the City as a free way to discard any combo pieces that get stuck in your hand.
Disclaimer: This is probably not the best shell for this deck, but the core combo seems strong and Gruesome Menagerie gives some protection. A self mill build may work better, or an abzan deck with a combo finish may be good.
