It's a pity this spell can't be cast at higher levels. 

Does the max health debuff stack or something?

The hp max deduction is 5th ed trying to mimic inconvenience of level drain. Vampires max hp reduction can’t be recovered until a long rest is taken. In this vein, the reductions should stack, and make a death blow a real possibility as reaching negative value of current max hp in one shot becomes achievable. 

If you want a deadly combo, inflict the CON disease (slimy doom) via Contagion, get at least 2 rounds of disadvantage saves for follow up harm spells. Bonus is Contagion is also a CON save at disadvantage. This can help burn through legend auto saves, setting up for Finger of Death or Disintegrate for a flashy ending. 

Another version of contagion is flesh rot, and makes target vulnerable to all damage types. This gives 2 rounds minimum of double Harm damage (as well as 3 rounds for rest of party). This is a solid conservative choice and would be the AL thing to do, but gives up % of doing a save or die versus a Legendary Creature and doing a mic drop and leaving the field of battle like a bawse.

Oops. Noticed this is only Cleric spell. Oh well, team up with mage. Might get 3 round minimum if you both either win initiative or cleric wins/mage loses or mage wins/cleric loses but mage delays til just after BBG.  

Why is this not a Druid spell?

Do you ever think “Spell names are too vague and/or confusing”? Well have I got the spell for you...

So if someone takes more damage than he has max hit points, he ends up with 1 HP and 1 max HP?

I believe if your max hp is reduced to 0, you die

indeed

Quote from Kevin_H >>

So if someone takes more damage than he has max hit points, he ends up with 1 HP and 1 max HP?

That's what our DM ran with when my character got wiped with this spell recently.

But it literally says in the text that it can't reduce your HP to below one, which I assume was put in place to allow DMs to use this spell without worrying too much about player deaths

I think you *could* manage to reduce a target's max HP below one with this spell, but it'd take a lot of setup work.

Let's say your target has 20HP and 10 temporary HP. They have 30HP but their "max hp" is only 20. Now if you hit them with harm for 25 damage, they lose 25 HP, so they have 5HP, but their "maximum HP" is reduced to negative five.

This is a bit of an odd situation, as the temporary hitpoints are always supposed to be lost first, but the wording of the spell effectively leaves the target with fewer maximum hitpoints than their current hitpoints, so it's a bit of a SNAFU.

(the sensible ruling is just to say that the temporary hitpoint damage doesn't reduce maximum)

If any effect that removes disease allows the hit point max to return to normal, would this mean the hit point reduction would not work at all on characters that are immune to disease?

chill touch. does not do cold damage and has a 120ft range cast and not touch

Lol, cleric using necromancy spells

death and grave domains

Question, do 3rd level paladin or a 10th level monk who fails a save against this spell, do they loose any hit points, as the effect states " Any effect that removes a disease allows a creature's hit point maximum to return to normal before that time passes." But they are immune to Disease from the beginning?

Quote from BoneWhale900 >>

Lol, cleric using necromancy spells

 Clerics are commonly associated with numerous necromancy spells - spare the dying, revivify, raise dead, resurrection, speak with dead. They've also got access to spells that create undead, steal life force, or inflict wounds/diseases/conditions.

No. Although it states that effects that remove disease can restore the HP max, as written, the HP reduction isn't a disease. It is a debilitating effect that results from failing the save against the spell, thus immunity to disease does not apply. 

Jeremy crawford has stated that immunity to disease equates to immunity to this spell entirely. You or your DM can do with that information what you want.

Jeremy crawford states that immunity to disease means immunity to this spell in its entirety damage included. If you are playing RAW or RAI then I'd say it has no effect. If you are one of the other boring, anti-fun, "you can't ignore a spell", people in the comments then I don't know what to tell you.

Quote from Cvr0109 >>

Quote from BoneWhale900 >>

Lol, cleric using necromancy spells

 Clerics are commonly associated with numerous necromancy spells - spare the dying, revivify, raise dead, resurrection, speak with dead. They've also got access to spells that create undead, steal life force, or inflict wounds/diseases/conditions.

 The spells you mentioned are meant for evil clerics.  Then again, a good cleric might use harm or inflict wounds as a representation of their god's might.  It's like saying, "you don't know who you are dealing with," or "you do not want to mess with me."  Or a good cleric could use harm as a curse for someone committing a taboo according to the cleric's religion.