Can Kai'Sa's legend ability be used to pay for the Deflect cost when playing a spell targeting a unit with Deflect?

Ruling: Yes, Kai'Sa's legend ability can be used to pay for Deflect costs. Since Deflect is a mandatory additional cost, it becomes part of the spell's total cost.

Sequence:

• Put the spell on the chain
• Choose targets
• Calculate and pay the cost (including any Deflect costs)
• Players can then react

Nuances:

• This only applies if the card being played is a spell (as required by Kai'Sa's ability)
• Deflect is not a triggered ability; it's an additional cost that must be paid when targeting