Does Kai'Sa's ability (which ignores power cost to play a spell) trigger Lux's ability (which triggers on playing 5+ cost spells)?

Ruling: Yes, playing a 5+ cost spell with Kai'Sa's conquer ability will trigger Lux Legends' ability to draw an extra card.

Nuances:

• The spell still has its original cost value even though you ignore the power cost to play it
• Kai'Sa's ability allows you to ignore the cost, but doesn't change what the spell's cost is for the purpose of other card effects