The wight has an attack called Life Drain:
Life Drain. Melee Weapon Attack: +4 to hit, reach 5 ft., one creature. Hit: 5 (1d6 + 2) necrotic damage. The target must succeed on a DC 13 Constitution saving throw or its hit point maximum is reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.
A humanoid slain by this attack rises 24 hours later as a zombie under the wight's control, unless the humanoid is restored to life or its body is destroyed. The wight can have no more than twelve zombies under its control at one time.
If the same wight hits a character with it multiple times, or a number of wights hit the same character with it, assuming the character fails their Constitution save each time, do the max HP reduction effects stack?
I know magical effects don't normally stack, but this isn't listed as being magical. Additionally, if it doesn't stack, I can't see how most characters could ever have their max HP reduced to 0.