But what's the difference? You agree with CustServ's response that any power of the form [Class] Attack [Level] is an attack, right?
Elemental Maw is an arcane power, so it's the next arcane power I use.
Elemental Maw is of the form [Class] Attack [Level], so it's an attack, so it's the next attack I make.
If the damage dealt by the effect becomes necrotic, then the damage dealt by the effect is damage dealt by Elemental Maw.
Since Elemental Maw is of the form [Class] Attack [Level], it's an attack, and it targets Reflex.
So we can take the Evasion wording:
When [an area or close attack targeting your AC or Reflex Defense] misses you but deals damage on a miss, you take no damage from [the attack].
and substitute in Elemental Maw where appropriate, since Elemental Maw is the attack:
When Elemental Maw misses you but deals damage on a miss, you take no damage from Elemental Maw.
And we've already established that the damage dealt by the effect is damage dealt by Elemental Maw... so when Elemental Maw misses you but deals damage on a miss, you take no damage from the effect.
Unless Elemental Maw is not, in fact, an attack, but rather includes an attack and an effect. In which case Wall of Fire is not an attack.
-Hyp.