We've brewed one of our sweetest combo decks of all time on day 1 of Theros Beyond Death being available on Magic Arena.
Grixis Storm Herald Combo
4 Sage of Mysteries 4 Hateful Eidolon 4 Dead Weight 4 Mire's Grasp 4 Mogis's Favor 4 Kaya's Ghostform 4 Storm Herald 4 Thassa's Oracle 4 Gruesome Menagerie 4 Swamp (339) 4 Blood Crypt 4 Temple of Malice 4 Watery Grave 4 Temple of Deceit 2 Temple of Epiphany 2 Steam Vents
This deck is definitely not straightforward, so we have some explaining to do.
We'll start with Sage of Mysteries, the innocuous but vital self-mill engine of the deck. We've been spoiled as self-mill fans between Arclight Phoenix and Kethis in recent Standard formats, and we've found another engine to build around with constellation.
Hateful Eidolon is our next key new card from Theros Beyond Death, although pretty much this entire deck revolves around new cards. Hopeful Eidolon is a cheap enchantment creature to trigger constellation that gives us an interesting method of assembling a card draw engine. We can play some enchantment based removal as our interaction, and it now doubles as card draw. Similar to Edgewall Innkeeper in the adventure decks, if you ever get multiples of this in play you get to go nuts. Mogis's Favor is especially awesome in this deck, since we're already planning to have a full graveyard. It pretty much never runs out of gas, meaning we can machine gun down opposing x/1 creatures or it can team up with another removal and draw multiple cards with Eidolon.
Thassa's Oracle and Storm Herald complete the puzzle. Storm Herald can be played for value or as a win condition. It can return any removal enchantments that have accumulated or been milled, and either draw or mill some cards. If you have Sage of Mysteries in play and a Kaya's Ghostform in the graveyard, you can return all of the auras in your graveyard, mill yourself with all of the triggers, and most likely find a second Kaya's Ghostform plus many more enchantments to keep the loop going a second time, third time, etc until the Oracle trigger that is still on the stack resolves. If you're playing on a digital client such as Magic Arena, be sure to be careful and adjust your settings so that you can stack your triggers manually.
It's somewhat reasonable to assemble this multi card combo due to the milling action and card draw present in the deck, but Gruesome Menagerie perfectly spits out our entire combo. It turns out that our combo is simply a 1, 2, and 3 mana creature, and the graveyard is where they're most likely to be. It's also extremely fun to see the look on your opponent's face when you're lucky enough to just cast this into an empty board on turn 5 and win the game after having just traded off a few pieces of removal in the earlier turns.
That's the list thus far! We were having some moderate success with a more "fair" version of this deck until we discovered Kaya's Ghostform and decided to go all in on the combo potential. If you're playing best of three, you'll likely want an alternate gameplan that can win through graveyard hate, or enough disruption to fight through it. There are also plenty of more sweet graveyard cards to consider tinkering with in Theros. Underworld Breach is particularly good in this deck since we can cast so many 1 mana spells that work best together, and there's a ton of escape creatures to consider. Stay tuned for more brews!