Does Polymorph break or melt Tomb of Levistus?

Question

Our group's paladin/warlock (multiclass) was wondering about the following scenario. The paladin/warlock uses his reaction to use the Tomb of Levistus eldritch invocation (from Xanathar's Guide to Everything, p. 57):

As a reaction when you take damage, you can entomb yourself in ice, which melts away at the end of your next turn. You gain 10 temporary hit points per warlock level, which take as much of the triggering damage as possible. Immediately after you take the damage, you gain vulnerability to fire damage, your speed is reduced to 0, and you are incapacitated. These effects, including any remaining temporary hit points, all end when the ice melts.

Then my character, our group's sorcerer, proceeded to cast polymorph on him immediately following. Does it break or melt the ice, immediately ending the effects of the invocation?

We do not know if there is an official answer to this, or how we are going to treat it just yet. When I cast polymorph on our barbarian who was raging to turn him into a giant ape, it was ruled that it ended his rage under the idea that polymorph changes your stat blocks and therefore ends class features, which include a warlock's invocations. We're not quite sure what to do here.

If a character is polymorphed after using Tomb of Levistus, what happens? Does the ice break/go away because the new form can no longer use the invocation?

Answer 1

Polymorph removes abilities, but not their effects.

Immediately after using tomb of Levistus, the warlock has some temporary hit points, and is subject to a magical effect that imposes vulnerability to fire, sets their speed to 0, and imposes the Incapacitated condition. The duration of this effect isn't connected to the warlock (it's not a concentration spell, for example), so there's no reason to think it would end for any reason before the stated duration (the end of the warlock's next turn).

Polymorph doesn't end magical effects; it replaces game statistics. Supposing that the warlock was polymorphed into a giant ape, they would then be giant ape encased in ice with speed 0, vulnerability to fire, and the Incapacitated condition. At the end of their next turn, the effects of tomb of Levistus would end as normal.

As for the temporary hit points, my reading of the effects of polymorph are that they would be lost. Polymorph says that the target "assumes the hit points of its new form", and temporary hit points are a special case of hit points. Since the new form has zero temporary hit points, those granted by tomb of Levistus would be removed when the warlock is polymorphed.

To contrast this with the example of the barbarian's Rage ability: Rage is an inherent ability of the barbarian, and once polymorphed, it no longer has that ability. Rage isn't an external effect applied to the barbarian:

Your rage lasts for 1 minute. It ends early if you are knocked unconscious or if your turn ends and you haven’t attacked a hostile creature since your last turn or taken damage since then. You can also end your rage on your turn as a bonus action.

(Barbarian class features, emphasis mine).

Note the use of "your". Once polymorphed, a barbarian no longer has a rage.

Answer 2

If you can be targeted by polymorph (depending on how you interpret the ice)...

Since the description of Tomb of Levistus says "you can entomb yourself in ice", this implies that you do not have line of sight to cast the spell, since the ice is "in the way", much the same as how you can't target someone through a closed window (mentioned in this answer to a different question).

PHB, pg. 204:

A Clear Path to the Target

To target something, you must have a clear path to it, so it can't be behind total cover, If you place an area of effect at a point that you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.

On the other hand, it doesn't say Tomb of Levistus gives you total cover, so this means that the ice could just be taken as a narrative effect that doesn't actually stop you from being attacked, in which case you could also be targeted by polymorph. Assuming this is the case...

Then there are the following possible outcomes

Since your warlock is under the effect of the Tomb of Levistus effect, and nothing about polymorph says it affects it, the polymorphed form is encased in ice and can move around as normal after it melts as per its description.

Either that, or the effects of the spells are postponed until the ice melts, similar to how polymorph and petrified condition might interact (see this question), although nothing about the effected being postponed is mentioned in either description, so this outcome doesn't have much support.

Finally, the Tomb of Levistus effect simply ends, since after being polymorphed, you are no longer a warlock, and therefore your invocation no longer applies. However, this also has the problem that Tomb of Levistus doesn't say anything like "humanoid" regarding a target like how a spell might be worded, it just says "you", and if you have been polymoprhed, you are still you, even if you are in a different form.

Given the above, I'd pick the first one I mentioned: you become a Giant Ape (or whatever) encased in ice until Tomb of Levistus melts as per its description.