I spent a lot of time making this and it underwent many variations before I concluded on a 'final build' of the list. It started with Caiphennadins, then underwent countless changes before I ended up with a list that no longer even includes Caiphus, adds black, and adds the Vara package.
The inclusion of black seems to make the deck so much stronger in my opinion. Vara is, at this point, one of the top 5 most powerful cards in the game, maybe even top 3. Any deck that can include her generally benefits immensely. One weakness of the Kennadins archetype from the very beginning has been the lack of lifesteal and healing, making it difficult to recover from a bad start against aggro decks.
Vara gives you a powerful recovery mechanic against aggro, a powerful blocker/removal fodder against midrange, and great pressure/threat against control. Most importantly, it acts as an amazing target for End of Hostilities. Essentially, Vara does absolutely everything this deck wants to do: Survive, apply pressure, and participate in the various tribute combos. She also removes aegis. Not only that, but Vara is an even more amazing target for End of Hostilities than one would assume at first glance. This is because when you copy Vara, it copies her at her current stats meaning that if she's a 5/6 when copied, she becomes a 7/8 (without the enemy sacrificing 2 units). If she's a 7/8, she becomes a 9/10. Essentially, Vara is one of the best Endless Hostilities targets in the entire game.
Madness is an incredible addition to the Kennadins lineup as well. Generally speaking, Combustion Cell in a traditional Kennadins list acts only as an engine for your lategame End of Hostilities or Kenna Grenadins combos. However, Madness gives Combustion Cell an additional purpose in creating a consistent way to use and then sacrifice your opponents creatures as a reliable form of removal + power.
Again, giving your combo deck even more combos is essentially the best thing you could ask for from this kind of deck.
Quarry and Devour, additions due to black are two more examples of amazing cards that fit right into the gameplan of the traditional Kennadins deck. Both of them offer you cheap draw mechanics which allow you to burn through your deck more quickly, finding the correct pieces needed to achieve the desired outcome.
Finally, the addition of black adds 2 new Crests to the list (8 power cards) which again, are crucial for scrying through your deck to find the exact pieces you need to win the game. I will reiterate that the addition of Devour, Quarry and 8 Crests allow you to find your combo pieces much more consistently and reliably than a traditional Kennadins list.
There are some drawbacks however. The most obvious being the inherent risks associated with running 3 color lists (getting mana screwed/influence blocked). This rarely happens, but is always a possibility in any 3+ color deck. Additionally, having 12 Crests helps quite a bit in finding the right pieces, but often delays you by one turn as opposed to a the traditional dual colored Kennadins deck which only runs 4 Crests. I find this drawback heavily mitigated in this kind of list however, because of the existence of Combustion Cell. Combustion Cell allows you to reliably generate 2 more power than you actually have, more than making up for the slight drawback of more Crests.
In the end, I removed Caiphus from the deck and replaced him with Scraptank. I did this for several reasons. 1. Caiphus is powerful, but not reliable. You can't control the keywords he gets, which means you can't rely that he will help you in whatever situation. 2. Caiphus is deleted in a single removal spell, Scraptank technically requires multiple removal spells unless its something like a Harsh Rule (which isn't always so bad as it's removing your opponent's board as well). 3. Scraptank acts as actual fuel for your Combustion Cell and endgame combos, Caiphus does not.
In the end both a great cards, but I find Scraptank to be a more reliable source of power, combo fuel, and a great End of Hostilities target. Scraptank can take over the game by itself. Even your opponent having a Torch in their hand is not enough to stop it. When you play Scraptank, if you immediately sacrifice a Grenadin with Combustion Cell, it will obtain 5/5 stats before the opponent has a chance to respond with removal making Torch useless.
Let's give an example of a common scenario: You play Scraptank on turn 5 with a Combustion Cell equipped. You immediately sacrifice one of the Grenadins it provides to give it 5/5 stats and giving yourself 2 power. You use the 2 power to play Strategize, Quarry, or hold Devour in case of a removal spell.
On the next turn, you sacrifice another Grenadin, giving Scraptank 7/7 stats, you play End of Hostilities on Scraptank creating 2 more 7/7 Scraptanks and 4 Grenadins, and then you swing in with the 7/7 Scraptank. Even if they kill it, now you have two 9/9 Scraptanks, which can be made into 11/11 Scraptanks on the next turn.
If left on board, the combo gets out of control so quickly that the opponent must answer it or concede within 2 turns. I sometimes play this deck against Clockroach and FTP Moment lists on ladder and laugh at their puny attempts to put huge threats on the board. By the time Clockroach decks are getting 4/4s out, I'm swinging in with 9/9s, it's an absolute slaughter.
So again, Caiphus is fun, and sometimes useful, Scraptank is much more reliable, and actually acts as combo fuel for the deck. Even if the Scraptank itself gets Permafrosted you don't care. It's still a perfectly good Hostilities target and Combustion Cell fuel.
This is one of the hardest control decks I've ever played. The stronger your opponent's threats are, the better your deck becomes. That's always been true of Kennadins to some extent, but even moreso with this list because of Madness allowing you to even use your opponent's threats, and Vara removing aegis on difficult targets like Icaria.
I don't know if I would consider it a tier 1 deck. It's very difficult to pilot (making the correct decisions with Crests/Strategize/Quarry requires a lot of experience and a consideration of mathematical probabilities). It still struggles against aggro lists (12 Crests, lack of cheap removal) and unitless control (endless Harsh Rules and End of Story is a problem), but against most traditional midrange and control decks, it has a huge advantage because it turns their strengths against them, which is the primary enjoyment of the deck.
Because of this, I consider it more of a tech deck. If you enjoy playing Kennadins-style lists, and aren't seeing an overabundance of hardcore aggro and unitless control in the ladder on any particular day, this could be the answer.