No, it is not a spell attack. In this regard, it is considered an untyped attack (it is most similar to a Master Assassinate, in this regard).
For clarity’s sake I will post Annihilation here:
the first function of Annihilation is that it is a Master*, Piercing** 100 (All) Bane*** attack that is also a Killing Blow****.
If Annihilation is used upon someone in any Death Count, they are instantly slain as though their Death Count has finished.
Lastly, it appears that Annihilation may have different effects on certain beings — that is to say, it may be even more lethal toward a specific type of entity or creature.
*Master-type attacks cannot be negated or defended by any Skill, Feat or Spell.
** Piercing attacks are considered to have hit their target even if it hits their Target’s Shield or Weapon. Similarly, defenses like Block or Parry are ignored by Piercing attacks.
*** Bane-type damage has no intrinsic bonus effect by itself. However, you cannot Minimalize (i.e. lower the damage to 1 damage) attacks that inflict Bane damage that is appropriate to your creature type (most PCs are Mortals, some are Spirits, etc). Furthermore, some creatures have additional penalties when being afflicted by Bane damage.
**** When you are hit by a Killing Blow and (1) are at 0 HP or (2) are dropped to 0 HP, you are brought to your Critical Death Count. Otherwise, it is a normal damaging attack. This means that if I receive a Killing Blow for 30 damage and I am at 5 HP, I am dropped to my critical Death Count.
"The End to War, the End to Pain,
these will come in Seventh's reign"