This is a budget Modern deck I built around Soulflayer. It came in around $30, which is super cheap for Modern.
Soulflayer was a card I was interested in working with back when it was in Standard, but it just didn't quite seem good enough. Cards like Gurmag Swiftwing and Chromanticore gave it some possibilities, but I never really got there.
The cool thing about Soulflayer is it gets better the more new creatures are printed. In Aether Revolt and Amonkhet, we got the first two creatures ever with both double strike and haste, which I felt was a big boost to Soulflayer because it made it much easier to have a Soulflayer attacking for 8 on turn 2.
Main deck:
Soulflayer - This is your main way to win, so getting a Soulflayer is super important.
Netherborn Phalanx - These are Soulflayers 5 - 8. The transmute ability lets you search up a Soulflayer. Since it's 3 mana, you don't get to cast SF until turn 4, but it's still way better than not having a SF at all. You'll rarely hard-cast the Phalanx.
Belltoll Dragon and Sagu Mauler - This is how we give SF hexproof. I tried putting both hexproof and indestructible in the deck, but in the end I felt like hexproof was more important, especially in game 1, so indestructible is in sideboard only. A nice bonus of these two creatures is you can play either of them as a 2/2 for 3 mana even though they're off color, then chump block to get them in the graveyard.
Samut, Voice of Dissent and Lightning Runner - As mentioned above, haste and double strike is a big upgrade for SF. And since Samut gives vigilance also, you end up with a monster on both offense and defense.
Vault Skirge - The last main deck creature to grant abilities, Vault Skirge is also castable as a chump blocker or early attacker.
Heir of Falkenrath - For a discard source that's also a threat, I looked at things like Cryptbreaker and Pack Rat, which can make a lot of power and take over the game on their own, but I settled on Heir because discarding for free balances well against it already being an ok creature in the early game.
Memory Sluice, Horrifying Revelation, and Corpse Churn all put cards in the graveyard for Soulflayer. Sluice gives you the most cards (and occasionally you might be able to double cast it), Corpse Churn has the upside of returning a SF that you accidentally milled, and Revelation lets you discard an ability creature from your hand.
Dakmor Salvage - You often and up milling lands. Salvage lets you still hit your land drops (albeit slowly) and it mills more cards in doing so.
There are certainly ways to make this deck better, especially by adding other colors of mana to it (green and blue add the most interesting milling/graveyard enablers, but to make the mana base work you're probably adding several hundred dollars in just lands), but in the budget version I kept it mono-black so that it can cast Soulflayer without problems. Generally strong Modern cards like Thoughtseize and Inquisition of Kozilek would also probably make it better.
Amonkhet brought us 5 indestructible gods with a second keyword, and I expect 3 more in Hour of Devastation. Maybe we'll get the Soulflayer holy grail of indestructible with hexproof? I would also love more 3-keyword creatures like Samut, and the best case for this deck is that they are overcosted or have other restrictions that make them unplayable in other decks. Spark Trooper (haste/lifelink/trample) and Drogskol Reaver (flying/lifelink/double strike) are two that are almost right for this deck.
First of all, the deck has a lot of variance in opening hands, so mulliganing is very important. Ideally, you want at least one Soulflayer or Phalanx in the opening hand, and at least one card that fills your graveyard, and two lands. A hand with, for example, Memory Sluice and Corpse Churn but no SF has reasonable odds to find a SF, but other hands without SF are probably going to automatically lose.
It's also important to figure out what the opponent is doing so that you can decide whether it's safe to play out non-SF creatures or whether you need to focus on getting SF as fast as possible, and whether you want to attack or hold back, and whether SF needs hexproof or lifelink to be effective. The optimal SF is made using Sagu Mauler, Vault Skirge, and Samut, for 7 keywords, but even a vanilla SF can be useful sometimes.
The sideboard right now has cheap removal, indestructible, and Vampire Nighthawks for a castable creature with lifelink and deathtouch. Without a ton of testing, the idea is that you might want to bring in indestructible/lifelink/deathtouch against decks where you will be playing more defensively. Hexproof avoids most of the main deck removal I would expect to see, but indestructible gets past sweepers that might come from the sideboard. Indestructible and deathtouch help get you past big creatures, as SF is never bigger than 4/4.
As for the removal, you usually get enough cards in the graveyard to cast Murderous Cut, and Sinister Concoction advances your plan the same way Horrifying Revelation does while also killing something. It's possible some removal belongs in the main deck, but there are enough dead cards and do-nothings already that I didn't want to add other cards that are potentially dead. Concoction can kill your own Vault Skirge if necessary, but that probably won't come up much.
I tried this deck out a bit and was surprised how well it played. It did lose a couple times on turn 3 and 4 to an aggressive prowess deck, but it also won a couple times. I feel like it will really struggle against decks with a lot of counterspells unless it can get the turn 2 SF. Since SF has natural resistance to Fatal Push and Lightning Bolt, you're pretty solid against removal. In most games, you don't really care whether other creatures survive as long as you can advance the SF plan.
Horrifying Revelation is sometimes better to cast on the opponent, like in burn type matchups where your opponent is going to slow way down once his hand is empty, or if you don't need it for yourself (e.g the rest of your hand is Memory Sluices and Soulflayers, or if you can use Heir of Falkenrath to enable your own discard instead). Memory Sluice should never be cast on the opponent normally.
It might be correct not to flip Heir of Falkenrath, depending on game flow. Since it's instant speed, you can always wait as long as you want.
If you play a morph creature face down, don't give away the fact that you can't flip it. Opponents might play around it until they figure out what you're doing.