How does spell cost reduction interact with cards that reference a spell's cost (like counters, Kai'sa's conquer ability, and Lux's legend ability)?

Ruling: A card's cost is always its printed cost, not the cost after reductions or modifications. Cost reductions do not change what other cards see as the spell's cost.

Sequence:

• When a card references "cost" it looks at the printed cost on the card
• Cost reductions (like from Sky Splitter's ability or Eager Apprentice) happen when playing the card but don't change the printed cost
• Cards that check cost thresholds (like Defy, Kai'sa's conquer, or Lux's legend) use the printed cost

Nuances:

• Sky Splitter can never be countered by Defy (which requires cost 4 or less) because its printed cost is higher, even though its energy cost may be reduced when played
• Kai'sa cannot play Sky Splitter from trash with only 2 power even if Sky Splitter's cost is reduced to 2, because the printed cost is what matters
• When Promising Future allows you to play a 5+ cost spell for free with Lux legend, you still draw 2 cards because the printed cost is 5+, even though you paid 0