Is there anything that can bypass temporary hit points?

Question

I have found a wizard spell from The Book Of Lost Spells, called Soul Shield. In my opinion, it is super overpowered, but my DM has allowed it.

The spell says that a wizard can convert their "real" hit points into twice as many temporary hit points, so long as you always leave yourself on at least 1 "real" hit point.

I instantly saw how amazing this was, however it leaves me wondering if there was a poison, disease, monster ability or the like that bypasses the buffer provided by temporary hit points and removes my remaining "real" hit point.
I have looked for such an ability, yet found none.

Is there one?

My DM likes to take things from homebrews quite a lot, so long as they are balanced, and I have had a look through a number of different sources. As such, a homebrew compendium is an acceptable answer for this question, but please link the source.

The spell in question works as follows:

Soul Shield

2nd Level abjuration

Components: VS
Casting Time: 1 action
Duration: 1 hour
Concentration: No

You siphon some of your life force into a shimmering shield of light around yourself. Sacrifice any number of your hit points (up to your current total -1) when you cast the spell, and you immediately gain twice as many temporary hit points. You must leave yourself at least 1 “real” hit point. While any of those temporary hit points remain, you get tactical advantage on Dexterity and Constitution saving throws. Remaining temporary hit points disappear when the spell ends. The spell ends immediately if all the temporary hit points are lost.

Answer 1

To my knowledge, no attack bypasses temporary hit points when dealing damage. However, some spells have effects based on the targets' hit points, and cannot be countered by having temporary hit points. For example:

• Sleep (L1 spell) only takes into account the targets' hit points, and can put a target into sleep much easier if they have low hit points remaining.
• Power Word Stun (L8 spell) automatically stuns a target with 150 or less hit points
• Power Word Kill (L9 spell) can instantly kill a target with less than 100 hit points.

Dispel Magic is also a convenient way to strip a character of most magically granted temporary hit points, leaving a character relying them vulnerable to reprisals.

Answer 2

The biggest threat to these temporary hit points as you purport to use them is time

There are several offensive effects that are limited to opponents with no more than a certain number of hit points, but they aren't the real threat here. The real problem is that, after one hour, you have both one hit point left and no temporary hp. Since you only have one hit point, you can't repeat the trick to cover yourself. If you have some method of massive magical healing, or better yet a way to top off for free after fights, you can heal yourself back up to avoid the almost-certain death from being at 1 hp inside an appropriately leveled dungeon, or you can be careful about the timing and limit your adventuring days to, say, 45 minutes, avoiding the spell when you know you'll have to press on for longer. Regardless, you will either be spending a fair chunk of healing resources or a huge chunk of time in recovery in order to survive this.

Also, if you do have the healing resources, there's no reason to wait till you are reduced to 1 hp to use them. You should heal up immediately after casting the spell, so you have twice your hp+8¹ (you did cast aid before casting this, right?) in temp hp for an hour, plus your normal hp after that. The spell is certainly ridiculously powerful, but leaving yourself with the possibility of suddenly being at 1hp without warning is a terrible threat that far outweighs the benefits, and there's no reason to assume that risk rather than just pouring in some healing effects.

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1. You must leave yourself at 1 hp so you can't double your last hitpoints. Aid increases your max hp by 5 temporarily. 2(X-1+5) simplifies to 2X+8.

Answer 3

I instantly saw how amazing this was, however it leaves me wondering if there was a poison, disease, monster ability or the like that bypasses the buffer provided by temporary hit points and removes my remaining "real" hit point. I have looked for such an ability, yet found none.

Several diseases, poisons, and monster abilities effect your maximum hit points. This ignores your temporary hit points. Most have pretty gruesome (read: instant death) effects if they reduce you to 0 hit points.

One example is the CR 1 Specter.

That maximum hit-point reduction probably doesn't scare you as an Ancient White Dragon with hundreds and hundreds of hit points, but just remember that threat level rises commensurately with power. If 40 specters all hit you with life drain and you fail a quarter of their saves, you'll take an average of 350 damage, ignoring, of course, the likelihood of one or more critical strike in that bunch.

I don't suggest that 40 specters are going to swarm your ultra mega dragon form, but there is precedent for swarms of ghosties killing large creatures. But the precedent is clearly there for damage that targets your maximum hit points, which is a way that you can be killed without burning through any temporary hit point shield.