Changes:
Protection Racket -> replace ?
Sulfurous Springs -> replace ?
Haunted Ridge - Slowland

Need:
$$ Bloodthirster - 6/6 flampler for 1 that can get me 4 combats per turn
$ Blightstep Pathway - Choose black or red depending on what I'm missing
$$$$ Archfiend of Despair - Wound reflection and anti-lifegain on one very discountable demon
$$$$$ Blightsteel Colossus - Combos with Chandra's Ignition, and is otherwise a strong threat
$$ Wandering Archaic - Good value creature that is castable even if I can't get Rakdos up
$$$$ Terror of the Peaks - Chunk people down every time they cast a spell, and act as an Warstorm Surge for large chunks later or a Rakdos-enabler with small creatures

Consider (don't have):
$ Rolling Earthquake - Hits opposing fliers, but also mine
Mortarion - Good with the Pestilence effects and Spear Spewer, as a mana sink to get a bunch of bodies for blocking or attacking
Tuktuk Rubblefort - Creature that gives haste
Frenzied Saddlebrute - Creature that gives haste as long as things attack my opponents
Descent into Avernus - Increasing board damage
Thrashing Wumpus - Pestilence on a 3/3
Kardur, Doomscourge - Demon that goads on ETB, and can gain some life later
$ Balor - "Cheap" demon with some utility attack/death triggers
$ Heartless Summoning - Could make things playable if I'm not allowed to have Rakdos, plus it lets me actually combo with Mikaeus+Ballista
$ Burning-Rune Demon
$ Viashino Heretic
$ Moraug, Fury of Akoum - More combats
$ Angrath, the Flame-Chained
$ Maga, Traitor to Mortals
$$$ Vial Smasher the Fierce
Raphael, Fiendish Savior - Give my other demons lifelink and make Rakdos into a 3 turn clock. Can also get some pinging devils if people start trading in combat
$ Master of Cruelties

Consider:
* Protection Racket - Sin Prodder trigger for each opponent, on an enchantment. I feel like this is likely to get me 1-2 cards each turn
Sower of Discord - Demon that gets me extra damage
Another Swamp
Arcane Signet - Ramp
Kroxa - Cheap way of getting some damage through, although if I really need it, people will likely discard, which is still decent. Also a way to use my graveyard
* Emrakul - Easy to cast and very strong, but quite feel-bad
Vandalblast - Sweep their rocks
Tibalt's Trickery - Counterspell
Dark Ritual - I need a lot of black mana, and this could let me get a turn 3 Rakdos
Desolation Twin - Can be free, and provides a huge body even through a counterspell
Syr Konrad - Anti-sweeper, but it can also enable casting Rakdos, and combo with Walking Ballista + Mikaeus
Court of Ambition - Chunk my opponents every turn, but they can avoid it by discarding cards. If I'm the monarch, this will probably always get the damage, but if I can keep the monarchy for a turn I probably have a decent board
Unlicensed Disintegration - Removal with Rakdos-enabling
Flame Rift
Knollspine Dragon - Refill my hand
Hurl Through Hell - Removal that can get me a threat
Varragoth, Bloodsky Sire - Demon that can tutor something to the top every turn, to make Sin Prodder and Keen Duelist really hurt, but needs to survive attacking
Voldaren Epicure - Enable Rakdos on the cheap and get some blood for future looting
Crashing Drawbridge - Haste-giver, but needs to be able to tap, so I can't use it for surprise haste
Wound Reflection
Ob Nixilis, the Fallen - A demon that can get really scary while also being an enabler for Rakdos
Hammer of Purphoros - Hasty fatties
Ogre Battledriver
Fervor - Hasty fatties
Brimstone Trebuchet
Rakdos, the Showstopper - If I go heavier on demons, this is both a one-sided sweeper and a beater, but still somewhat unreliable
Ob Nixilis, Unshackled - People would either have to be stupid to search with this out, or they're searching for something that's going to make it not matter. I guess it stops people from doing the small searches, or prevent game-ending tutors to snatch away an almost-win. Also, growing when things die is nice.
Rakdos's Return
Persist - Reanimate a fatty
Bontu's Monument
Lord of the Void
Valakut Awakening
Pyroblast
Shizo, Death's Storehouse - Evasion on a colored land
Cauldron Familiar (+ Witch's Oven)
Syr Carah, the Bold
Torbran, Thane of Red Fell
Xantcha, Sleeper Agent
Garna the Bloodflame
Impact Tremors - Play a small dude pre-combat to enable Rakdos, and enable Ancestral Statue combo
Captive Audience
Yidaro - 8/8 trampler for 2 possibly, with the option of cycling early and possibly getting it again later
Toil // Trouble
Crypt Rats
Endless One - Scalable, free fatty
Carnival // Carnage
Kediss, Emberclaw Familiar
Painful Lesson - Card draw or ping on the cheap
Wretched Confluence - Card draw that can ping, or other utility
Cauldron Dance - Get back a big dude (like Ulamog) to my hand and either sneak in some blockers or get two hasty attackers
Implement of Combustion - Cheap ping that replaces itself
Risk Factor - Can either draw or ping, but I don't get to choose. Re-use is nice, though
Collective Defiance - Wheel away spare lands etc, or ping, or remove a dude or all of the above
Incendiary Command - Flexible spell that can ping someone and wheel
Void
Sunbird's Invocation - Get some more value out of the big dudes I cast
Etali, Primal Storm - Value fatty
Ulamog's Crusher, Breaker of Armies - Fatties that can be free
Chandra, Pyromaster
Mirage Mirror - Extra copies of good fatties or other useful stuff
Morbid Curiosity - Refill hand by saccing a fatty
Faithless Looting - Card selection for cheap to fix a bad draw
Ayara, First of Locthwain
Revenge of Ravens
Embercleave
Rankle, Master of Pranks
Crimson Mage - Hasty fatties
Sepulchral Primordial
Havoc Festival - Great at shortening games, and it hits me last, so if it gets removed, I probably don't care that much
Chandra, Awakened Inferno - Deals damage on the wrong turns, but fits the theme of grinding down everyone's lifetotals like Mogis
Mogis, God of Slaughter - I need the damage on my own turn, but it's still good for the "Rakdos chipping away at your life totals" theme
Erebos - Card draw and make sure people don't gain life
Retreat to Hagra - Helps me win the race by gaining some life, while also enabling Rakdos
Torrent of Souls, Animate Dead, Exhume - Reanimate some big dudes
Scion of Darkness - Reanimate on a fatty
Dread Cacodemon - Boardwipe fatty
Baleful Force
Gruesome Slaughter, Burning Anger
Molten Influence
Cabal Paladin
Lightning-Rig Crew
Pharagax Giant
Scythe Specter
Sizzle
Words of War
Fateful Showdown - Wheel away some lands or useless cards and let me cast Rakdos
Nihilistic Glee - Pinger that can draw me cards if I run out
Orcish Cannonade
Pyrite Spellbomb
Grave Consequences - Enable casting Rakdos or exile all graveyards
Phyrexian Etchings - Risky card draw
Psychosis Crawler

Lands:
Wastes + Evolving Wilds and Terramorphic Expanse - For Endbringer
Reliquary Tower
Lantern-Lit Graveyard
Myriad Landscape
Sea Gate Wreckage

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Lands to cut:
Rakdos Guildgate - The flavor is so nice, but always entering tapped for no benefit is too rough

Considered cuts:
Thermo-Alchemist / Nettle Drone - Not enough untaps, but might be needed to get an early Rakdos
Anger - I don't have a lot of ways to put this in the yard
Terminate, Dreadbore, Bedevil - I will probably have the biggest dudes, so might not need so much spot removal
Harvester of Souls - Makes it safer to throw down a bunch of fatties risking a board wipe, and happens to be a scary fatty himself. Doesn't really help me get out Rakdos though
Rakdos Charm - Flexible and flavorful, and can stop some critical things such as infinite tokens or a huge reanimation effect, but is it worth a card? The fact that it can allow me to cast Rakdos might make it worth it
Conduit of Ruin - Only tutors for a few select bombs, but it does help me discount a dude each turn for when I don't have big daddy R
Cryptborn Horror - A decent body for those turns where Rakdos would have been sweet but I'm not allowed to cast him
Sin Prodder - Giving the choice is not great, and this will likely lose me any cheap cards I need if I play it early
* Demonlord Belzenlok - Could actually kill me
Earthquake - Can't be discounted
Fire Diamond - ETBT and doesn't produce black
Wayfarer's Bauble - Ramp that won't be susceptible to a late game sweeper, but won't let me get a turn 3 Rakdos
Neheb - Back-up Rakdos, but I still need to be able to dish out some damage, and this doesn't help me with black. Purphoros might need it more
Oblivion Sower - Not guaranteed to get me any lands. Becomes a lot better with Bojuka Bog, but probably still needs to be pretty late in the game
Plague Spitter - 3 mana for 1 round of pings per turn might be worse than some of the other pingers that can untap, but also continually sweeping dorks is nice
* Pestilence Demon - Very heavy black mana requirements, and if I already have Rakdos out, I don't need this for pinging. It does however let me make 3 mana for every black mana I have if I have Rakdos but can't attack for some reason
Walking Ballista - Might not be worth it when it cannot combo
Pestilence, Pyrohemia - Mana intensive, and if I lose Rakdos after a sweeper, these will also go away
Meteor Golem - Is only a 3/3, so this basically just ends up being a removal spell that can potentially be free with Rakdos, and otherwise probably a bit too expensive

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Cut lands:
Temple of the False God - I have a lot of pips and not a lot of land ramp
Ramunap Ruins + deserts - 5 mana and saccing a land is a steep cost just to enable casting Rakdos
Bloodfell Caves - ETBT, and 1 life isn't worth it
Akoum Refuge, guildgate - ETBT
Leechridden Swamp - I need to already have 2 black permanents to get the ability from this, in which case I probably don't have a huge issue enabling Rakdos. Not worth the ETBT
Ghost Quarter? - With this heavy color requirements, I might need to let other people deal with problem lands

Cuts:
Breath of Malfegor - Too much mana for only hitting players
Deceiver of Form - I have so many other big fatties that the ability might not do anything, and without the ability he's not that great
Gibbering Fiend - Delirium is probably hard to get
Fanatic of Mogis - Probably won't have a lot of devotion to red
Lobber Crew - Not enough untaps
Stormbreath Dragon - Monstrousing is expensive and might not do a lot, and this is just a "small dragon". It is an evasive hasty creature to get that lifeloss for Rakdos, though
Ill-Gotten Inheritance - Allows me to cast Rakdos any turn, as well as break parity, but it's a little expensive
Ob Nixilis of the Black Oath - Quite expensive for the minor drain, but the ult is amazing if I can ever get there
Acidic Soil - Can make most things free with Rakdos, and can punish green players, but it's risky as a late game ramp card as it can potentially hurt a lot
Sheoldred - This isn't really a reanimator deck
Grenzo, Dungeon Warden - Scalable size, and can get me more dudes if I cast him as a big/big, but he'd be much better if we had some reanimation and deck manipulation
Kozilek, the Great Distortion - Double colorless mana can be tricky
Pathrazer of Ulamog, Bane of Bala Ged - Goodstuff to discount, but they do nothing besides having (pseudo-)annihilator
Sanctum of Ugin - I only have 5 cards left that can trigger this, and slightly more it can search for. I'd rather have a colored source
Damnable Pact - Mana inefficient, but somewhat flexible
? It That Betrays - Annihilator is unpopular. Getting some extra stuff is nice, but if I have this and I'm attacking, I'm probably doing fine. Not requiring any colored mana is very relevant, though
? Indulgent Tormentor - Don't want to give them the choice, and with 3 toughness this is relatively easy to kill. It is a demon, though, and I can pick whoever is most likely to give me the result I want (probably the one that lets me cast Rakdos)
Endbringer - I currently only have 7 sources of colorless in the deck, which isn't really enough to reliably use this
Staff of Nin - Expensive to get started with
Disciple of Bolas - I very rarely want to use this, so might as well take this out for Bolas
Dire Fleet Ravager - Might hurt me too much
Urabrask - Besides being a powerful creature, it doesn't synergize a lot since I will be casting most of my creatures post-combat
Skull Rend - Might be too expensive for the effect?
Inferno Titan - The ability is mostly useful to enable Rakdos, but as a 6-drop it might be a bit too slow for that
Cut // Ribbons - Ribbons can only be use offensively, and probably not safe to use except when finishing someone off
Crypt Ghast - Might not have enough swamps for this to be worth a slot
Steel Hellkite - I can discount the dude, but not the ability, and it only destroys with CMC exactly X and only from the attacked player, so this is unlikely to get more than either a board of tokens or a few rocks
? Emrakul - Mindslavering is really good, but also very feel-bad
Vicious Rumors - Enables Rakdos for cheap, but might not be worth a card
Chandra, Torch of Defiance - The first ability allows me to cast Rakdos, and the ultimate is bonkers, but I'm probably not going to be able to defend this for long and 4 mana for 2 damage is a bit much
Bloodfire Colossus - Sweeper that can be discounted, but will also kill most of my board (including Rakdos) and hurts myself
Last One Standing - Cheap boardwipe, but it's random
Chainer, Nightmare Adept - I'm not really reanimating things in other ways, so the haste clause is limited. Is trading cards in hand for cards in graveyard once per turn worth a slot?
Exsanguinate - Can't discount, so Phage probably needs this more
Breath of Darigaaz - Less flexible Earthquake
? Burnished Hart - Rocks are probably just better, as this will still always cost 3 mana to ramp with. It is however decent insurance against a Rift or similar, allowing me to redeploy more easily
Warstorm Surge - 6 mana for a non-discountable win-more. If this is doing relevant amounts of damage, I'm already landing great creatures
Chandra's Ignition - Too narrow! Does nothing when I'm behind and can't keep a fatty around, and if I can, this will probably kill most of my own board
? Mikaeus - Very nice as sweeper protection, but it doesn't actually go infinite with Walking Ballista because of the lord ability, and BBB is rough
? Heartless Hidetsugu - Might hurt me too much
Rakdos Locket - The worst of my ramp