During the event period, when you spend 1 Gold or Coupon you'll get 1 Reiryoku point. When you collect enough Reiyoku points you'll get corresponding coupons as gifts (sent immediately via mail). The progress will be cleared everyday at Midnight.
Spending 8000 Gold or Coupons will get you 2175 Coupons in return.
During the event period, when you spend 1 Gold or Coupon you'll get 1 Kasö-ryoku point. When your Kasö-ryoku point reaches corresponding amount then you can receive the corresponding Hollow King reward. The progress will stay until the event is over.
Prizes from September 5, 2016 (click to enlarge):
Prizes as of August 7, 2016 (click to enlarge):