Mythos of Vadrok

• WWizards of the Coast2020-04-17
  Official Ruling
  The target creatures and planeswalkers will be unable to attack, block, or have their abilities activated even if the damage that would be dealt to them is prevented. However, if one of those permanents becomes an illegal target, it will be able to attack, block, and have its abilities activated.
• WWizards of the Coast2020-04-17
  Official Ruling
  Each chosen target must receive at least 1 damage.
• 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
  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
  If some of the targets become illegal for Mythos of Vadrok, the original division of damage still applies, but the damage that would have been dealt to illegal targets isn’t dealt at all.
• 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
  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.
• WWizards of the Coast2020-04-17
  Official Ruling
  Activated abilities contain a colon. They’re generally written “[Cost]: [Effect].” Some keyword abilities are activated abilities and will have colons in their reminder text. Triggered abilities (starting with “when,” “whenever,” or “at”) are unaffected by Mythos of Vadrok.

Rulings via Scryfall