Dormammu got revealed by Atomic Mass Games, and he has a few abilities that needed some maths doing on them to put them in context. So here’s the numbers:

The first thing to look in to is his Spender attack:

The trigger requires a critical, hit, block and fail. Four specific results on 8 dice is unsurprisingly not that common. It comes out to 28% chance to hit once, and so only 8% to get three attacks off.
For context, Sabretooth trigger his spender 56% of the time, and Loki triggers his 21% of the time. Having played a bit of Sabretooth, you typically don’t expect the trigger on the spender, and you certainly shouldn’t rely on it. Loki’s trigger is pretty rare, and so this feels like a “great when you get it” effect, but like a lot of characters, this spender is one that likely won’t be used that much.
Hits
With a 6 and 8 dice attack, Dormammu at first glance doesn’t seem that great on the damage dealing scale, especially for 8 threat, but that’s before you start factoring in the second half of his Master of the Realm of Darkness superpower:

Changing the maths on the dice is really good, as anyone who has played much Corvus Glaive or Scarlet Witch will tell you. But stealing extra successes from your opponent’s defense roll adds another layer. The more dice they roll, the more likely it is that you will get extra successes. As such, I’ve had to calculate the chances of hits against different defense option.

As you can see, his 6 dice attack varies between “about 8 dice” to “almost identical to 9 dice”, depending on their defense.
It’s worth noting at this point that defensive rerolls don’t change this maths, as – out side of a few notable exceptions like Miles’s leadership and Doctor Strange – fail results can’t be modified.
His 8 dice spender attack is, understandably, even better:

It compares very closely to “reroll any” attacks, so if you are looking for a shorthand, this power makes his 8 dice spender attack strongly comparable to an 8 dice reroll any attack.
That feels more like an 8 threat!
Damage
Hits is interesting, but as defense will scale with more dice, you need to factor in the defense rolls and look at damage. So I did:

From my own anecdotal experiences and from talking to people online, 4 damage seems to feel “about average” on Dormammu’s “builder”, and the maths almost supports that, or at least shows why you might think it. It’s slightly lower against typical defenses, but the amount of incinerate that he throws out shift the odds in his favour.

His 8 dice attack is better – again to no surprise – but for 5 power and only a 28% of another attack, you have to ask if it’s worth it. I think the answer will be “rarely”, but not never; especially if you look at the odds of chaining two builder attacks together:


It’s going to be pretty common for Dormammu to daze/KO a character if he gets to make two attacks against them. He’s going to feel a lot like reality Corvus, but from Range 4! Immunity to stagger makes it more likely, as does his super power that places him. Add in movement from Tactics Cards or superpowers on other charcaters, and it’s that much more likely.
Anyone who has tried him and feels his attacks are lacking, that can totally happen in this high variance game, but I would strongly recommend trying him again: he really does feel like an 8 threat.