Mythos of Nethroi

• WWizards of the Coast2020-04-17
  Official Ruling
  Mythos of Nethroi can target any nonland permanent. Determine as the spell resolves whether the permanent is destroyed, either because it’s a creature at that time or because was spent to cast the spell.
• WWizards of the Coast2020-04-17
  Official Ruling
  If an effect copies the Mythos spell, no mana was spent to cast the copy, so the copy won’t receive the bonus.
• WWizards of the Coast2020-04-17
  Official Ruling
  If an effect allows you to cast a spell without paying its mana cost, you can’t choose to cast it and pay unless another rule or effect allows you to cast that spell for a cost. Similarly, you can’t waive a cost reduction unless that effect says you may.
• WWizards of the Coast2020-04-17
  Official Ruling
  The ability checks what mana was actually spent to cast a spell. If an effect allows you to spend mana “as though it were mana” of any color or type, that allows you to spend mana you couldn’t otherwise spend, but it doesn’t change what mana you spent to cast the spell.
• WWizards of the Coast2020-04-17
  Official Ruling
  The abilities of the Mythos check what colors of mana were spent to cast the spell. It’s not an alternative cost to cast the spell.

Rulings via Scryfall