When does hp max decrease occur for Incubus’s Draining Kiss?

@BryceWaltonWhen does hp max decrease occur for Incubus’s Draining Kiss? i.e. Ra has 9/9 hp, takes 4 damage. Now Ra has:
a) 5/5 hp
b) 1/5 hp 5/5

— (((Mike Mearls))) (@mikemearls) July 7, 2016