I sort of see this Assault as a sort of "Holodeck," it says you walk in and the area will change every time you go in; so it like puts you in whatever area the mobs you end up fighting are from - thus answering why Fafnir or Behemoth, etc, are in ToAU. And doesn't thorougly discount sky gods (they're the Zilart expansion too, just like Kings) from being there, though a six person Seiryu/Byakko will certainly be interesting. 6 person Kings doesn't entirely frighten me, I've seen it done.
What if you walked in and it was King Ranperre's Tomb and you found yourself face to face with Vrtra.... :D
Also, I recall Salvage also said "There are no requirements" in the update announcement, and then we found you had to be past Black Coffin, so obviously there is something off in translation or what they are considering "requirements," if they mean tags/points/rank etc vs actually being at a certain point in missions.
EDIT: best possible fight to walk into - finding yourself in an instanced Ru'Avitau with Ullikummi wandering around. I would possibly cry in happiness - except my static Assault group is 5 people from one HNMLS and then me from a separate.
