Tethmos High Priest

• WWizards of the Coast2014-04-26
  Official Ruling
  Heroic abilities will trigger only once per spell, even if that spell targets the creature with the heroic ability multiple times.
• WWizards of the Coast2014-04-26
  Official Ruling
  Heroic abilities will resolve before the spell that caused them to trigger.
• WWizards of the Coast2014-04-26
  Official Ruling
  Heroic abilities won't trigger when a copy of a spell is created on the stack or when a spell's targets are changed to include a creature with a heroic ability.
• WWizards of the Coast2018-12-07
  Official Ruling
  If a card in a player's graveyard has in its mana cost, X is considered to be 0.

Rulings via Scryfall