When Sett enters the field with one buff (making him Mighty) and you spend that buff to activate his ability granting +4/+4, does he lose and regain Mighty status, allowing yellow Fiora to ready him?

Ruling: Sett will lose Mighty status when you pay the cost to activate his ability, then regain Mighty status when the ability resolves.

Sequence:

• Sett enters the field with one buff, making him Mighty
• You pay the cost (spending the buff) to activate his ability
• Sett ceases being Mighty at this point
• The ability resolves, granting +4/+4
• Sett becomes Mighty again

Nuances:

• Because Sett loses and regains Mighty status during this process, yellow Fiora can ready him when he becomes Mighty again