Can Kai'Sa's legend ability (Exhaust: Reaction – Add (1 power). Use only to play spells) be used as the first thing on your turn, or does the Reaction keyword require something else to be on the chain first?

Ruling: Reaction means you CAN play it in reaction to something, not that you have to. You can use a Reaction any time you could play an Action or a unit, including as the first thing on your turn.

Nuances:

• Kai'Sa's ability only pays for spells, not for other costs like hiding cards