Spawn of Mayhem. Is. Awesome
I fully expect Spawn of Mayhem to be a major player in Standard, as it's an interesting card if you don't try to play to its strengths and merely accept a four-mana 4/4 flying, trample, and whatever you want to call that final ability. But like everyone else, when I saw Spawn of Mayhem, I saw a deckbuilding challenge to get it on to the battlefield on turn 3 via spectacle as often as possible.
If my main goal is enabling spectacle early in the game, this likely means a Mono-Black Aggro shell with Diregraf Ghoul and Graveyard Marshal, or I could easily see wanting to be Rakdos Midrange to splash Bedevil. If I were to try to add Spawn of Mayhem to an existing strategy, I think this card would fit perfectly in Golgari Midrange. I think cheap "throwaway" creatures are one of the best ways to enable spectacle, and specifically, Merfolk Branchwalker and Jadelight Ranger can produce value for you before they get into combat and enable spectacle.
Three mana for a 4/4 flier with trample is already a great card, and that doesn't factor in its final ability enabling spectacle in all subsequent turns. Not only is it enabling spectacle but it's also just dealing direct damage to each player outside of combat. When you read Spawn of Mayhem, you leave with a feeling that it's this awesome cheap 4/4, which it is, but the fact that it deals that extra one damage turns it from a five-turn clock to a four-turn clock, then even faster once you factor in that it's also picking up extra +1/+1 counters later.
For Standard, Spawn of Mayhem strikes me as a remarkably powerful card against Jeskai Control. Living through Deafening Clarion is already a massive upgrade on many of the three-mana creatures black has in Standard, but the interaction I enjoy the most is the benefit you gain from having it on the battlefield. Usually if you have a huge creature and your opponent has mana available for Settle the Wreckage or Seal Away, it's a tough and problematic situation since you want to force the issue and deal as much damage as possible, but there's always a fear against control that playing into their reactive cards allows them to be mana-efficient.
On the flipside, waiting around can be worrisome since all control wants is time to develop. Spawn of Mayhem helps quite a bit since by not attacking, you still have the option to passively deal one damage per turn. This may not seem impressive, but I believe it's highly relevant for this particular matchup because it allows my aggro deck to have a cheap threat and a direct damage burn card if the situation calls for it.
I could almost feel the light bulb appearing above my head when I read it for the fifth time and realized its final ability is rewarding Spawn of Mayhem's controller for having a low life total, not the opponent. This is slightly unusual, at least to me, intuitively speaking, since it's a big bad Demon and you'd think it's bloodthirsty and the opponent's low life total would be the reward. But no, the Cult of Rakdos rewards you for damaging yourself.
You could try all varieties of two-color aggro or midrange decks with Spawn of Mayhem, but unless you're truly mono-black, you could have a few shocklands in your deck in Overgrown Tomb or Blood Crypt, which now have added strategic value since if you play against control you could just pay all your life to all your shocklands, almost like a Death's Shadow deck might to get below ten life and start picking up extra +1/+1 counters on each Spawn of Mayhem you control. Maybe this exact interaction could end up deciding who wins in a mirror match when both players control Spawn of Mayhem.
Obviously, I love Spawn of Mayhem, but there are still plenty of great options to kill it:
Lava Coil, Conclave Tribunal, and Justice Strike all immediately come to mind, and even just writing down the names of those cards I also felt obligated to mention Mortify and Bedevil. And let's not forget about Ravenous Chupacabra, a card that Spawn of Mayhem is straight card disadvantage against. It's always tough to try and place an evaluation on how good a creature is when the opponent doesn't kill it or how powerful the effect is if you only have it for one or two turns before it's killed.
At three mana, Spawn of Mayhem is a totally acceptable if not an outright good investment for an aggressive deck, and midrange decks should be very careful. If I was a Rakdos Midrange deck and I had to choose between Spawn of Mayhem and something like Rekindling Phoenix, it's hard for me to resist the insurance the Phoenix provides. I could easily be wrong and you could easily see them played alongside each other, but I'm nervous for a future with a lot of best-of-one competitive play where having a threat that's versatile against a wide open field may be better than short-term raw power.
Spawn of Mayhem reminds me a lot of Desecration Demon, a card that's near and dear to my heart. It's not fancy and you don't need some nuanced strategy to win with it; it's just raw power and stats paired with mythic rarity and sweet artwork, and sometimes that's more than enough to get the job done. I'm excited to see which decks adopt Spawn of Mayhem, and I'm incredibly optimistic about its place in the Standard metagame. It almost feels like something crazy would need to go wrong for it to not be widely played and successful. It has everything you might want out of a huge early threat, and it even matches up well against Enigma Drake and Crackling Drake, two cards that seem to be everywhere.
Like I said at the top - Spawn of Mayhem. Is. Awesome.