Clarification of "spirits" on "Devouring Greed" card
After a little more research and finding some loosely related info on another question here I found the following in the rules.


  302.3. Creature subtypes are always a single word and are listed after a long dash: "Creature -- Human Soldier," "Artifact Creature -- Golem," and so on. Creature subtypes are also called creature types. Creatures may have multiple subtypes. See rule 205.3m for the complete list of creature types.
  Example: "Creature -- Goblin Wizard" means the card is a creature with the subtypes Goblin and Wizard.


If a creature has a subtype of "demon spirit", it is a "demon" and a "spirit" which I believe satisfies the "spirits" requirement of "Devouring Greed".