For the Historic portion of the Strixhaven Championship I found myself registering Five-Colour Niv-Mizzet. This was a bit of a surprise to me, as a few days earlier I'd found myself sheepishly asking one of my teammates, "Would you humour me and let me test this against you for a bit?" I'd assumed all these decks I'd been beating on ladder would crush me in focused testing against a good player and that I could quickly move on and just register Izzet Phoenix or Orzhov Auras myself. But even in focused testing I just couldn't stop winning, and importantly I became very confident that I was favoured against the aforementioned Izzet Phoenix with Niv-Mizzet. The showing for the deck was so convincing that both my teammates, who were skeptical at first, ended up jumping onto the deck themselves late into the testing process.
During the Historic rounds of the event, I was paired against Izzet Phoenix in all seven rounds, which is fairly absurd. One of those rounds I didn't get to play due to internet outages, but in the other six I had a combined record of 12-1 in games; my thinking that this deck was able to crush Izzet Phoenix was proven true.
It's not that surprising that Niv-Mizzet Reborn would be good in Historic. Really it was only a matter of time; the card has been great in both Pioneer and Modern for a while, after all. The deck had been explored previously in Historic to middling success, but a few new multicolour cards can make all the difference, and it turns out we gained access to some great ones recently.
Stirxhaven brought a pair of amazing removal spells for the deck which really upped Five-Colour Niv-Mizzet's ability to efficiently answer opposing threats. The lifegain offered by Lightning Helix is huge when you're mainly trying to tread water until you can resolve the titular Dragon. Vanishing Verse exiles, which is great against a threat like Arclight Phoenix, whilst also offering an answer to scary enchantments and planeswalkers like Trail of Crumbs or Chandra, Torch of Defiance. Both these cards have weak spots, but they solve enough problems that they really help round out this deck's arsenal. They're even two-mana instants, which means they work fantastically alongside Teferi, Hero of Dominaria!
The real gain for the deck though was Expressive Iteration. I'm not sure I've played another deck in Historic that's so good at taking advantage of that card. Your land count is high, meaning that if you want to use Iteration to hit your land drop you're quite likely to hit; in this situation your 27-land deck is 84% to find at least one land, whereas Izzet Phoenix with its 21 lands is only 73% to find one.
Meanwhile, thanks to Thoughtseize and Inquisition of Kozilek, you also have a lot of efficient spells to cast off Iteration on Turn 3 as well; if you already have an untapped black source in hand, and you assume you're happy to either hit your land drop with Iteration or find a discard spell for this turn, you're now at 92% to get that full two cards of value. It's very rare that Expressive Iteration doesn't do a good Divination impression in this deck.
Even better than this, Iteration is also just structurally a helpful card to the deck. It digs deep looking for whatever colour of mana you're missing, whilst also digging to find your Niv-Mizzet since you really want to resolve that card every single game; the fact that Niv-Mizzet can then grab you an Iteration, which in turn can dig you towards your next Dragon, means the card advantage often just won't stop rolling in. Even better, in other decks taking two lands off an Iteration is often fine but unexciting, but here, not only are you looking to hit many, many land drops, but with eleven Triomes in your deck there's a decent chance that you get to cycle one of those lands you take off Iteration as a rebuy.
The other big thing helping Five-Colour Niv-Mizzet is the discovery of how you want to build this Historic archetype. The thing is, you don't actually need every Niv-Mizzet trigger to draw you four cards. Sure the times when you do the game will just be very over, but a 6/6 flyer that draws two good spells? That's just a great card. Honestly there are times where your Niv-Mizzet can grab you a single spell and that will be good enough. This means you don't need your deck to consist exclusively of multicolour cards and as a result you get to play Thoughtseize, one of the most powerful cards in Historic.