Annihilation-Class is a descriptor used by the Rapid Evening for beings and objects considered dangerous to the galaxy at large (or at least the status quo). The term is mainly used in Twilight Mirage for Independence and Volition's Axioms, though it's likely past entities such as Rigour would also be placed into this category.
Some highly advanced technology is categorized as Adversary-Class (perhaps items similar to what was previously called Rigour tech or Rigourkin in COUNTER/Weight).