Can the hit point maximum be reduced repeatedly with multiple Harm spells?

Question

I know that the effects of a spell don’t combine if it is cast multiple times. Only the most potent effect would be applied. In our last game, this topic came up in regards to the Harm spell. Harm was cast on an opponent. He failed the save, took a lot of damage and his hit point maximum was reduced drastically. A turn later, he got slammed again with Harm. This time, he succeeded on the save and very little necrotic damage was dealt. Then, the discussion started whether his hit point maximum would now be further reduced or if the “more potent” Harm spell takes precedence.

Naturally, I argued that his hit point maximum would not be reduced further, because effects of the same spell cast multiple times don’t stack. The counterargument was, that the necrotic damage dealt by the Harm spell is clearly an effect of the spell. And no one would argue, that the damage dealt by two consecutive Harm spells would only be applied once. So now I ask you:

Can the hit point maximum be reduced repeatedly with multiple Harm spells?

Answer 1

The target's maximum hit points will only be reduced once

From Harm (PHB, pg. 249):

If the target fails the saving throw, its hit point maximum is reduced for 1 hour by an amount equal to the necrotic damage it took. Any effect that removes a disease allows a creature's hit point maximum to return to normal before that time passes.

Combining Magical Effects, from the PHB (pg. 254):

The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect - such as the highest bonus - from those castings applies while their durations overlap.

So multiple castings of the same spell will not stack, but rather the one that dealt the largest amount of damage would take precedence. This rule applies because the hit point reduction has a duration attached to it, unlike, say, a Wraith's Life Drain (MM, pg. 302), which is an ongoing affect (albeit with an end condition):

... its hit point maximum is reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest.

Of course, this doesn't stop the target's maximum hit points from being reduced further by something other than the Harm spell...

This is essentially the argument you presented at your table, but I see no reason that this wouldn't apply.

Answer 2

I agree... the necrotic damages applied as direct damage would be applied for each instance (essentially being two separate attacks which caused damage), however, I would say that if the player failed both saving throws the bigger of the two would be applied and the duration would be modified appropriately (depending on which one was the larger).

However, since in this case you said that one failed and one passed, then by rule, you'd only reduce the maximum hit points once.

True, too, the Wraith's Life Drain is a byproduct of it's special attack rules (a spell-like effect)-- those would and could stack -- in fact, that I think is why a Wraith is designed to be a tougher opponent.

And, yes, other spells/spell-like effects could effect maximum hit points negatively (or positively - e.g. temporary hit points)