We don't know the power of his decree to give a definite explanation, but even if we did and the aforementioned curse was similar to Galan's or Merascylla's, I wouldn't expect a logical explanation from Nakaba's side.
I think maybe a former fairy king.
A Fairy and maybe a Human.
But... whom? We saw dolor and even in spite of the extra set of limbs and weird skin color it seems like his basic appearance is the same. Assuming the same applies to a fairy king then the implication would be that whoever this is would have a glorious set of wings. So we can rule out anyone who in fact does not have wings. For anyone here to be a fairy at all it would have to be someone who could potentially hide his wings. Which leaves only two choices, the black blob and octololi. We would be assuming either of them is hiding wings under the black mass or the tentacles.
The black blob definitely(we don't have a nickname for him yet?).
I am not sure it is entirely certain dolor will be dealt with by diane. Take galan. How many vendettas did he earn? He petrified merlin and beat the crap out of mel (though he did return the favor). Merascylla did everything to earn ban's literally eternal hatred. And even then both of them got annihilated by escanor, a random dude they met by chance in a cave in the middle of nowhere. Sure, the fight would make sense but in the end I don't think the author is aiming to be predictable in this manner.
With the buildup, it's almost as inevitable as Meliodas VS Zeldris IMO. In the case of Galan and Merascylla, they wer mostly trying to run away. I am pretty sure they will come back too, even if we consider Merascylla dead, it's not the same case for Galan.
But Dolor says they have fallen. His eye allows him to know exactly what's going on, so it's very likely they have been dealt with for good.
At this point I just can't see Diane not being the 1 to beat Doler. He most likely kill Matrona as well.
Well, he said they've been defeated in MC translation, and despite his powers he doesn't know where Fraudrin is, there are limits. At the very least, Galan is in the same state as Merlin so he isn't dead.
How do we know merlin would be alive if she didnt transfer here soul?
I think of petrifaction as simpy incapacitation pending when the effect is nullified or removed but the idea probably varies based on who (author not character) is using it in their work. There's really no way to tell but seeing as Galan proceeded to break the petrified form after (albeit it being a fake), that probably indicates that the petrified person needs to be destroyed or they'll live if the spell is removed.
How powerful is Fraudin in his real form compared to the other Commandments?