Your Graveyard only has to be empty when you activate “Dimension Distortion”, so if an effect is chained that puts a card in your Graveyard, “Dimension Distortion” still resolves.
“Dimension Distortion” can only Special Summon a monster that is face-up while removed from play (cannot Special Summon a monster removed with “Lightforce Sword” or “Different Dimension Capsule”).
“Dimension Distortion” can Special Summon “Dark Ruler Ha Des”.
When a “Special Summon-only” monster is removed, without returning to the hand or Deck, after it is Special Summoned properly, it can be Special Summoned by “Dimension Distortion”.