This is my take on The Scorpion God as a commander. As soon as this guy was spoiled, a million ideas flew through my head. Finally we have a commander that can turn the great -1/-1 counter support that Rakdos provides into pure card advantage! In addition, he by default ignores commander tax outright with his third ability. Handy! I look forward to building the deck.
First thing to note is that I've gone for somewhat of a combo focus. The combo at the forefront of the deck is Blowfly Infestation + Nest of Scarabs. With Scorp out on the field, and provided you can generate one measly insect with his ability, you can then use Scorp's ability to kill that insect with a -1/-1 counter, which will both produce a new insect off the effect of Nest of Scarabs, but the insect dying will then let you put a -1/-1 counter on the new insect. This can loop infinitely, and we have a multitude of ways to capitalise off this loop.
First and foremost, with The Scorpion God out on the field, this combo nets you infinite card draw which you can stop any time by simply putting the Infestation -1/-1 counter on a different creature than the insect token. You can use this card draw to drill down for any of our game ending combo pieces - Blood Artist and Zulaport Cutthroat can wipe your opponent's life totals out in the blink of an eye, you just have to be careful you don't deck yourself out in the process with The Scorpion God's effect. As such, it might be a good idea to kill him off with one of our kill spells before popping off with this combo. Deathbringer Thoctar will get infinite +1/+1 counters from this combo and can then proceed to burn your opponents out in the same manner as Blood Artist and Cutthroat.
We also have a lot of tutoring in the deck to help grab these combo pieces as soon as we can. The obvious ones such as Demonic Tutor, Diabolic Tutor are present of course, but the lesser known Diabolic Intent pertains well to our game plan by both tutoring for combo pieces, and providing a sac outlet to draw cards off should they have a -1/-1 counter on them. Planar Bridge provides completely repeatable tutoring at a reasonably high price, and playing the card puts a pretty big target on your head. Black classics Read the Bones, Sign in Blood, Phyrexian Arena and Underworld Connections aid in drawing cards to get to the answers and combo pieces we need.
Outside of comboing off, we have a number of great ways of abusing Scorp's ability to spread -1/-1 counters like no tomorrow (aided by the effect of Heartstone). Crumbling Ashes cleans the board out while also netting card draw while Scorp is out. Mikaeus, the Unhallowed provides for some dirty revival of your creatures, and you can then annihilate the +1/+1 counters with -1/-1 counters to keep your creatures coming back over and over again. Liliana, Death Wielder also helps with both distributing -1/-1 counters and destroying creatures that are already crippled by them. A particularly gross combo in the deck is Carnifex Demon and Sudden Spoiling - a combo which reduces your opponent's creatures to 0/2, then the -1/-1 counters that you already spread around with Carnifex will instantly wipe out somebody's board, getting around even hexproof and indestructible since there's no targetting and the death will be the result of state-based action. Cauldron of Souls is also crazy in the deck since you can force your opponent's creatures to come back into play with a -1/-1 counter on them even if you couldn't get to it first. What's more, if you give persist to a creature with 1 toughness, they'll instantly die again and net you more card draw! Also, Kulrath Knight turns Scorps' ability into a makeshift Pacifism.
It's barely worth going into how many additional ways we have of spreading our -1/-1 counters, but Incremental Blight, Midnight Banshee and Carnifex Demon are a few standout examples.
In addition to all the counter fun, we have ways of capitalising off the death of not only our own creatures but the opponent's as well. Black Market can very quickly get out of hand and provide you with a crazy amount of black mana every main phase. Ogre Slumlord is an interesting creature that provides us with handy deathtouching chump blockers which can also be hit with a -1/-1 counter for more card draw. Harvester of Souls also provides a bunch of card draw for capitalising off the litany of death we'll be causing.
The more generic Rakdos kill options are also here to help finish off limping creatures - Dreadbore, Terminate, Hero's Downfall and Murder are all solid spot removal. Damnation, Spontaneous Combustion, Blasphemous Act and Toxic Deluge round out the package with the mass removal that commander requires.
For the situation where swinging face is the best option since you already cleared out the opponents' boards of any threats, Glistening Oil and Fists of the Demigod turn The Scorpion God into a very scary bringer of death that can even get around indestructible creatures. Skithiryx, the Blight Dragon functions as a great alternate win condition should all else fail.
I've seen a lot of other people focus their Scorpion God builds around Everlasting Torment and Soul-Scar Mage and including a lot of burn to spread -1/-1 counters in that way, but I steered away from that since I feel that building 30% of your decklist around abusing the effect of two cards is a little too rail-road-y for my taste, so I went for the more combo-based route. This might seem paradoxical, but I've tried to include plenty of ways to win and abuse Scorp's effect without relying entirely on the combo.
