Kaervek the Merciless

• WWizards of the Coast2021-03-19
  Official Ruling
  For spells with in their mana costs, use the value chosen for X to determine the spell's mana value.
• WWizards of the Coast2021-03-19
  Official Ruling
  An ability that triggers when a player casts a spell resolves before the spell that caused it to trigger. It resolves even if that spell is countered.
• WWizards of the Coast2021-03-19
  Official Ruling
  Alternative costs, additional costs, and cost reductions don't change a spell's mana value. Its mana value is still based on its mana cost.

Rulings via Scryfall