Strategy
This deck is Rakdos incarnate. The whole point of this deck is to accelerate the game as fast as possible, even if it means you might not be the last player standing. This deck carries just as much regard for your own life total as it does your opponents; just try to make sure everyone else is losing life at a faster rate than you.
The best cards for doing this are your damage doublers. Dictate of the Twin Gods, Furnace of Rath, and Wound Reflection all pair up beautifully with forcing your opponent to attack, your multiple sources of persistent damaging effects, and big game-ending cards like Mob Rule, Insurrection, and Exsanguinate.
If you can get your opponents' life totals low enough in the first place, damage doublers aren't even needed! Cards like Heartless Hidetsugu, Dire Fleet Ravager, Fraying Omnipotence, Pox, and Havoc Festival get your opponents to a low enough life total that you can win without the help of damage doublers.
So in short: deal damage, deal more damage, deal even more damage, double the damage if you need to, and try not to die from all the damage you're dealing. Follow this simple plan and you should be able to win!
Punishment
The meat of the deck. As mentioned in strategy, this deck slings a lot of damage around; even to yourself. The whole strategy is to get your opponents to a lower life total than you and keep them there until they're out of the game. Below are some key cards that will help you do exactly that.
Ankh of Mishra & Zo-Zu the Punisher - Reliable damage while punishing decks who try and ramp by playing more than one land a turn.
Manabarbs - Similar to Ankh of Mishra, it punishes decks that want to ramp with lands and play large spells while offering consistent damage across the board.
Spellshock - Reliable damage that punishes opponents for playing storm or aggro-style decks.
Rampaging Ferocidon - Unlike the above two cards, the damage this card deals is much less reliable because your opponents' decks might not be too creature heavy. Even though the damage isn't as reliable, the potential of this card against token/aggro decks could be devastating while not effecting your life total that much.
Price of Glory and Citadel of Pain - Punishes anyone trying to play instant-speed spells, but is mostly in here as defense against counterspells and end-of-turn boardwipes. This deck doesn't run too many instants and, as a result, rarely needs to leave any mana open. We could either go the route of incorporating more instant speed interactions, or we could try and bring our opponents down to our level. In the spirit of punishing opponents, I love this card.
Pyrostatic Pillar, Eidolon of the Great Revel, and Spellshock - Primarily here to punish aggro and storm decks; any deck that wants to play a lot of cheap cards in a single turn is not a fan of these three cards.
Harsh Mentor - Punishes... well... pretty much everything. Doesn't target any specific strategy aside from maybe a "ping" deck, but just an all-around good card to include for this deck.
Mana Accelerators
Since Xantcha has an activated ability that both deals damage AND draws us cards, we want to use it as much as possible. Here are a couple of cards that help us do that.
Braid of Fire - With the removal of mana burn, this card is just pure upside! The only reason it doesn't see much play is that you can only use the mana this generates on your upkeep and many decks aren't built to utilize this. The best usage of this card is when you have a mana sink in your command zone (which is why you mostly see it in Grenzo, Dungeon Warden decks); thankfully, we have exactly that.
Neheb, the Eternal - What is this deck trying to do again? Oh yeah, a lot of damage. Neheb generating mana based on damage dealt means you can invest that mana into dealing even more damage! Whether that's in the form of dumping it into Xantcha's activated ability or playing some more punishing cards from your hand is up to you since this mana (unlike Braid of Fire) can be used during your second main phase.
Coldsteel Heart, Fellwar Stone, Rakdos Signet, Sol Ring, Star Compass, Talisman of Indulgence, and Victory Chimes - Your standard ramp cards. Helps us to get to larger amounts of mana much more quickly. Victory Chimes in particular also helps your opponents take advantage of Xantcha's activated ability as well!
Combat Manipulation
Xantcha has the ability to make its controller attack every turn if able. While this theme isn't fully explored to its max potential in this deck, I definitely wanted to make sure I included some cards that help synergize with that gameplan.
Disrupt Decorum - Possibly my favorite card in MTG history. This card not only forces your opponents to attack, but forces them to attack EACH OTHER. Really solid card and even though the price is starting to creep up on it, I feel it's massively underplayed.
Rite of the Raging Storm - Similar to Disrupt Decorum this card makes your opponents attack each other and you give them the fuel to do it with. It kind of acts like a second Xantcha without her activated ability. Just be wary that you don't play this on the same turn you play Xantcha. If you do, make sure you give Xantcha to the player on your right. That way you reduce the risk of having another player attack the Xantcha controller with a lightning rager; giving Xantcha's controller a chance to block with her.
Goblin Spymaster - Makes your opponents attack, but be wary because they can also swing at you. Make sure you're at a high enough life total, your defenses are up, or you're not the biggest threat at the table before playing this card.
Sleeper Agent - Truthfully, this card is mostly in here for the flavor. That being said, it's another card that, even though it doesn't explicitly force them to attack, it highly encourages them to. Similar to Goblin Spymaster, don't play it if you think you're going to be on the receiving end of the attack.
Insurrection and Mob Rule - If your opponents won't attack with their creatures, you can attack with them instead. These cards are primary win conditions in the deck, especially when paired up with one of the damage doublers.
War's Toll - Makes Xantcha's controller also swing with everything else they control. Has the added upside of also making sure our opponents are usually tapped out at the end of each of their turns.
Removal
This might feel like it conflicts with the "Combat Manipulation" section. Even though we want our opponents swinging big creatures at each other as much as possible, we also want to be able to make sure that things don't snowball out of hand. Us having enough removal helps us ensure that things stay chaotic without getting too terrifying.
Last One Standing - One of my favorite boardwipes ever printed. 3 mana to clear everything except for one random creature? Given the amount of creatures played in a standard game of EDH, the one creature standing is almost never a downside. In the best of cases, you could randomly choose a creature that you control!
Blasphemous Act - Another super cheap boardwipe that should clear practically everything aside from indestructible creatures. This card will almost always cost under 4 mana whenever you want to cast it.
Toxic Deluge - Another cheap boardwipe (are you sensing a trend here?) and arguably the best boardwipe in EDH aside from maybe Cyclonic Rift.
Fraying Omnipotence & Pox - Not quite a boardwipe in the traditional sense, especially since the creatures you're trying to destroy will most likely stick around. That being said, it still reduces their board state, lowers their life total, and makes them discard hands; all things this deck wants to do.
Terminate, Dreadbore, Unlicensed Disintegration, Dismember, and Kolaghan's Command - All cheap single-target removal cards. Great for taking out problematic utility creatures or some scary creature heading your way. Also can be used to take out Xantcha if she ends up back under your control (aside from Kolaghan's Command).
Draw
Much like ramp and removal, card draw is one of the essential pillars of any deck. Unforunately, red doesn't draw cards in the traditional sense (looting & impulse draw) and black draw wants you to pay life. Both aren't ideal for this type of deck, but here are the best ones that I could find.
Necropotence - Even though this deck doesn't have a lot of life to play with, Necropotence is just SO good that it's hard to make an argument for not including it. It gives you so much card advantage that it feels like a win condition on its own.
Outpost Siege - Choosing Khans gives you access to reoccurring impulse draw. Because our CMC is relatively low, you should be able to cast whatever card is flipped if you want to. The biggest downside is that it triggers on upkeep; meaning you don't get any benefit from this card the turn that you play it.
Phyrexian Arena - Not the biggest fan of this card since it waits until your upkeep to gain any advantage from it (similar to Outpost Siege) and it loses you life. Both are pretty large downsides for this deck, but given the options that we have access to in black/red, it's one of the better ones we can choose.
Xantcha, Sleeper Agent - Our commander itself is a draw engine! Especially when paired up with cards like Braid of Fire or Neheb, the Eternal she helps you draw cards when you need them most.
Night's Whisper - One-off card draw. Not the most attractive option but can help out in the early game and make your three-land-hand a little more attractive. Also not an awful topdeck because it replaces itself while drawing another card. Trying to remove one-off effects, but, as I mentioned many times earlier, finding good card draw in these colors that doesn't cost massive amounts of life is difficult.
Throne of the High City - Monarch can be a way to get your opponents to start attacking each other and can be a source of card draw. I feel that it's better for forcing attacks than card draw, but could be a potential source of draw.
Updates
Small update — Aug. 28, 2018
It's been a little bit since I've updated this deck, and for good reason; the deck is running great! As I mentioned in the comments, it's not a competitive deck by any means but works perfectly in the way that I was hoping it would. I was scared that taking out Pyrostatic Pillar and Eidolon of the Great Revel would leave me without enough damage. Fortunately, the deck still seems to get along just fine without them (and my life total has never been happier). Before I ramble on too much, here are the new cuts and additions:
Cuts: Expedition Map, Night's Whisper, Indulgent Tormentor, Unlicensed Disintegration
Additions: Thran Turbine, Syphon Mind, Mind's Eye, Chaos Warp
Cuts
Expedition Map - This card was primarily put in the deck to fetch Glacial Chasm or Maze of Ith when I needed the damage mitigation or to tutor up a land drop in the early turns. Since then I've removed Glacial Chasm so the only real utility land for it to tutor up is Maze of Ith. Also I've kept Expedition Map in an opening hand before and it never felt great to tutor up a Graven Cairns or something like that. If I decide to put in Cabal Coffers and Urborg, Tomb of Yawgmoth I will probably end up putting this card back in.
Night's Whisper - Fantastic card for its CMC, might even be better than the Syphon Mind that I'm replacing it with. In the end, it pretty much just comes down to preference. Having a draw spell that only costs 2 CMC usually gives you the opportunity to play one of the cards you drew whereas a 4 CMC spell might not. Syphon Mind, however, works with the discard theme of the deck and drawing that (usually) extra card can make a large difference. I feel that Night's Whisper, Read the Bones, and Syphon Mind can all be pretty interchangeable here and running multiple, all, or none of these cards could also be the correct answer. I mostly just want to test out Syphon Mind and a card had to be cut.
Indulgent Tormentor - The cards that give your opponent two terrible choices always sound incredibly fun, but if both effects aren't equally punishing then you end up with an underwhelming card. Take Dash Hopes for example. No one would play this card because losing 5 life in EDH isn't that big of a deal whereas having your spell countered can potentially be game-endingly bad. That's kind of the same issue with Indulgent Tormentor. All three of its effects are pretty amazing on their own. You're essentially getting a Baleful Force, Sheoldred, Whispering One, or Sulfuric Vortex kind of effect. The problem is that your opponents choose the effect; and they will always choose what's best for them and worst for you. It also has anti-synergy with our commander because they can choose the "sacrifice a creature" option to just sacrifice Xantcha before she can deal a lot of damage to them.
Unlicensed Disintegration - Not a bad card by any means, but I feel that it just gets outclassed by the great single target removal that Rakdos has access to. Terminate and Chaos Warp are pretty much auto-includes, Kolaghan's Command has fantastic utility with its flexibility, and Dreadbore (despite being sorcery speed) is just another fantastic single removal card similar to Terminate. An argument could be made for Unlicensed Disintegration over Dreadbore but I like the option to be able to use it against planeswalkers if needed. Also an argument could be made to include both cards, but finding cuts in this deck is getting harder by the day.
Additions
Thran Turbine - Really fun card that I've never seen played before. Lets us activate Xantcha's ability for 1 mana once a turn or for free if we also control Heartstone.
Syphon Mind - Covered it pretty extensively when talking about Night's Whisper above. Pretty much a preference choice but I like the way it synergizes with the discard theme of the deck.
Mind's Eye - Great card that strangely also synergizes with the deck. Everytime our opponents activate Xantcha, we are able to pay 1 mana to also draw a card. If you consider that, plus the card they draw on their draw step, plus any additional draw effects they might have in their deck, this card should draw you a fair amount of cards. I am a bit worried that having this on the field would discourage people from using Xantcha's ability, but that's why we playtest!
Chaos Warp - Don't know how I managed to forget this card. Rakdos is wonderful at removing creatures & artifacts, but not enchantments. Chaos Warp is pretty much the only card we have to really deal with enchantments (excluding Enchanter's Bane which I am looking to playtest too) so it should definitely be included. Also has the added benefit of using it on our commander if she gets returned back to us for added value.
So those are the current cuts and additions, but here are a few cards that I'm looking at playtesting and cutting in the future. Potential additions
Platinum Emperion, Basalt Monolith, Rings of Brighthearth, Enchanter's Bane - These are all cards suggested in the comments below that I haven't gotten a chance to try yet! I need to get my hands on a copy of these cards but might just end up proxying them so I get the chance to give them a whirl before I buy.
Claws of Gix - It may not be the best sacrifice outlet for the deck, but I am considering adding in sacrifice outlets and the flavor is too good. Also considered Unnerve for the flavor purposes but I think that might be going a bit overboard.
Torment of Hailfire - I think I took this card out of the deck too quickly. I didn't like that they could sacrifice Xantcha to its effect, but it is so powerful that I think it might earn its spot back. Especially with the inclusion of Mana Geyser and the potential inclusion of Basalt Monolith + Rings of Brighthearth
Bojuka Bog or Planar Void - I feel I need more graveyard removal aside from just Rakdos Charm but not quite sure yet.
Ramp - After removing Expedition Map, Thautmatic Compass, and Mind Stone, I'm concerned with the deck's early game ramp. I'll have to play a few games to see if it's still at a good amount.
Potential Cuts
Sleeper Agent - I adore this card for the flavor, but in gameplay it hasn't been so kind. If I give this card to someone in the early game they attack me with it and if I give it to someone later in the game they just attack someone with a big creature who can destroy it. I want to like this card, but it hasn't really done wonders for me yet.
Vial Smasher the Fierce, Virtus the Veiled, or Tree of Perdition - Lumping these cards all together even though they're very different. They all fit into the category of "low cost, potential high impact creatures". So far they have been removed before I've gotten to do anything with them but I think that speaks to how good they are instead of how bad they are. Will keep an eye on these cards but I've practically not been able to play them yet!
Pox or Fraying Omnipotence - Love the effects of these cards but not sure if I need two of the similar effect. Also not a fan that they can sacrifice Xantcha to these cards' effects.
Angrath, the Flame-Chained - Still in the early stages of playtesting, but so far it's been "okay". Not amazing, not bad, but okay. Probably won't make the final cut of the deck, but it's definitely fun to try out for the time being.
Harsh Mentor - Discourages our opponents from activating Xantcha's ability. Even though it would technically make the card worse, I wish the card didn't include activated abilities of creatures too.