Say Hello to the Adze of Annam, the New Hardest-Hitting Weapon in 5E

 Ever since Mythic Odysseys of Theros came out, I've looked to Akmon, Hammer of Purphuros, as the hardest-hitting weapon in the game. It's an artifact that is best if you're a worshipper of the eponymous deity (Theros' version of Hephaestus, basically) and is a +3 warhammer that does 3d10 additional fire damage on a hit, along with a few other bonuses.

As far as I can tell, it was the weapon in all of (officially published) 5E that hit the hardest, hitting for 1d8/1d10 + 3d10 damage, plus 3, before your own personal bonuses. So, two-handed, that's 25 damage on its own with each swing. Notably, the Sword of Zariel from Descent into Avernus actually keeps pace with it when wielded two-handed (and is also the basis for the artifact my party has acquired in my Ravnica campaign - basically the redeemed sword of a champion of Serra from Dominaria that has a living angel inside - the sword's stats are slightly tweaked, but the damage is the same). But one-handed, Akmon does a little more, as its 3d10 remain the bonus while the Sword of Zariel's radiant damage is downgraded to 2d8.

But Bigby Presents Glory of the Giants has brought us a new king of weapons: the Adze of Annam.

The Adze is a +3 Greataxe that does an additional 3d12 force damage, meaning we're looking at 29 damage on average before we take the wielder's strength into account.

The Adze does have a few other bonuses, namely being able to cast Move Earth or Fabricate once a day (and it seems that using one locks you out of the other for the day) along with a couple minor beneficial, minor detrimental, and one major beneficial traits you can randomly roll. But that's about it.

I'd still say the Sword of Zariel is probably the most powerful weapon they've ever made, due to the massive number of additional features it provides. But if we're talking about pure hard-hittingness, this is it.

Now, I'll note that there's actually a nonmagical weapon that comes close to this. In the DMG, amongst the futuristic weapons, the antimatter rifle deals 6d8 necrotic damage on a hit. That's an average of 27 damage per hit, meaning that if you had a +3 magical version, it would actually out-damage the Adze. The Antimatter Rifle, however, has the reload property, which the Gunner feat notably does not let you ignore. Thus, you get two shots and then must use a bonus action or action to reload the weapon. While other reload weapons often have a clip of several shots - the laser pistol, for example, can fire 50 times before reloading, meaning it's unlikely you'll ever have to do so in the heat of combat - this limitation on the antimatter rifle makes it less ideal for the "max damage scenario" fun of a level 20 fighter. Technically, you could get four shots off in a round, but you wouldn't be able to do so again the next round.

If we had a Champion Fighter with maxed out Strength using this agains the Tarrasque, we'd do the following damage on average:

We have a +14 to hit, meaning we hit on an 11 or higher, so 50% we're hitting overall.

Hit damage is 34

With a 15% chance to crit for an additional 26 damage.

So, per attack, we're looking at 17 plus 3.9 for crits, giving us 20.9 damage per attack, and thus doing 83.6 damage per round. (And unlike with Akmon, all our damage actually affects the Tarrasque).