So here's a deck I've been playing on Magic Arena lately.
I don't claim this is good, but it is funny, and it'd make an even funnier Commander deck. This is one of those combo decks where self-destruction is very much a possibility. Here's the basic premise.
You don't have to play a mutate card onto a real creature – you can do it on a token. This deck has a bunch of instants and sorceries that make a token (plus Emergent Sequence, which animates a land). Once Illuna mutates onto a token, it triggers, exiling cards from the library until it hits the only nonland permanent in the deck: Omniscience.
There are many ways this can break. You can draw Omniscience (at which point you can either put it back on top with Brainstorm, or discard it to one of various rummaging/looting effects and shuffle it back in with various graveyard-shuffling spells). Or Omniscience can simply be too close to the bottom of the library, leaving you with not enough cards left to win the game. Much like casting Ad Nauseam, this combo can instantly lose the game if your library is stacked in an unfortunate way. In fact it's way less consistent than casting Ad Nauseam. But let's assume you put Omniscience into play without exiling too much of your library. That's only the first half of the combo.
From there, you want to win the game on the spot because letting your opponent untap in Historic Brawl, even with an omniscience in play, is very bad. The way this deck actually wins is by playing one of several token making effects that make extremely large tokens...
The deck is filled with draw spells, including expensive ones that can draw seven cards or more; once Omniscience is in play it's generally possible to draw a large portion of the remaining library to find the last few combo pieces. Body of Research is nice because assuming there's enough library left, it's basically guaranteed to make a lethal-sized token. Serpentine Curve similarly will make a lethal-sized token almost always, as it counts not only the graveyard but also any spells that Illuna exiled on its way to Omniscience.
But the army token is the best one, because you can make an Army token ahead of time, mutate Illuna onto it, cast Commence the Endgame to make it gigantic, then attack with it. It's also possible to recur Commence the Endgame and cast it multiple times to stack more counters on the same token. Illuna conveniently has flample, so a sufficiently large Illuna-fied army is guaranteed to get through for lethal damage regardless of board state – though it's important to remember that you want to mutate Illuna under the army, so the resulting creature keeps the Army creature type and thus receives the +1/+1 counters from Commence the Endgame.
However, in many cases you won't be able to go off with an army, and so you need to turn the gigantic creature into damage immediately. This can be done by granting it haste with Maximize Velocity or by pitching it at the opponent with Fling; Kazuul's Fury is a nice Fling variant that's also a land MDFC.
Besides that, the deck plays as much efficient ramp as it possibly can get its hands on in order to go off on turn 5 instead of turn 6, including lots of treasure generating spells. The only interaction here are things that are "free" (cards like Flip the Switch that make a token while being interactive) or a small handful of spells meant to protect the combo, like An Offer you Can't Refuse – this deck probably should also play Pact of Negation and Swan Song, but I need to figure out what to cut.
