Arla's God's Art will still trigger even if Arla is no longer on the field. If this happens, Arla will still be the source of damage, whether as a Ruler or Astral Ruler.
Arla's God's Art deals damage in stacks. The first attack causes Arla to deal 400 damage to a single target resonator, the second attack causes Arla to deal 800 damage to a single target resonator, and so forth.