Does a spellcaster know when their spell has no effect due to a save or immunity?

Suppose a cleric casts Zone of Truth on a captured enemy. Unbeknownst to them, the enemy is a rakshasa. Zone of Truth says (PHB, p. 289):

You know whether each creature succeeds or fails on its saving throw.

But (MM, p. 257):

The rakshasa is immune to spells of 6th level or lower unless it wishes to be affected.

Assuming the rakshasa doesn’t wish to be affected by the Zone of Truth spell, does the caster know this?

More generally, does a caster know when targets are unaffected by a spell they have cast, either because they have made saving throws, or because they are outright immune to the spell or its effects (e.g., casting Ray of Sickness on a shadow demon, which is immune to poison damage and the poisoned condition)?

What happens when a spellcaster with Legendary Resistances Shapechanges into another creature with Legendary Resistances?

A few high level arcane spellcasting monsters, such as the Lich from the Monster Manual, also have Legendary Resistances (here, 3 per day).

Such spellcasters can prepare then cast Shapechange on themselves to become a legendary creature (such as an Ancient Brass Dragon), while keeping some things:

You retain the benefit of any features from your class, race or other source and can use them, provided that your new form is physically capable of doing so.

It is clear from the Monster Manual‘s errata that there are several things not gained from assuming the form of another creature:

If a creature assumes the form of a legendary creature, such as through a spell, it doesn’t gain that form’s legendary actions, lair actions, or regional effects.

But there is no mention of Legendary Resistances here.

What happens when a spellcaster with Legendary Resistances casts Shapechange on itself to become another creature that also happens to have Legendary Resistances?
(For the example’s purpose, let’s say that the Lich has used 1 of its 3 resistances before Shapechanging into the Dragon.)

  1. The caster’s legendary resistances (here, 2) get overridden by the resulting creature’s legendary resistances (here, 3) but become available again when the spell ends
  2. Both sets of legendary resistances are available at the same time (for a total of 5)
  3. Some other scenario

Does a spellcaster with Lair actions keep them when Shapechanging into an Ancient Metallic Dragon who then uses Change Shape?

A few high level arcane spellcasting monsters, such as the Lich from the Monster Manual, also have Lair Actions.

Such spellcasters can prepare then cast Shapechange on themselves to become an Ancient Metallic Dragon (such as a Brass one), while keeping some things :

You retain the benefit of any features from your class, race or other source and can use them, provided that your new form is physically capable of doing so.

It is my understanding that the above would allow the Lich to keep its Lair actions in dragon form.

Such dragons have a special Change Shape ability to transform into a humanoid or beast (such as a rabbit) while keeping certain things :

[…] retains its alignment, hit points, Hit Dice, ability to speak, proficiencies, Legendary resistances, lair actions, and Intelligence, Wisdom and Charisma scores, as well as this action.

It is clear from the Monster Manual’s Errata that the Lich could not gain the Dragon’s Lair actions (or the resulting beast/humanoid lair actions, if any) :

If a creature assumes the form of a legendary creature, such as through a spell, it doesn’t gain that form’s legendary actions, lair actions, or regional effects.

But what I’m wondering here is whether the now-twice-transformed creature would retain its initial form’s Lair actions. In this example, would the Lich-transformed-into-an-Ancient-Brass-Dragon-transformed-into-a-Rabbit keep its Lich Lair actions ?

Can a spellcaster use a focus not specified by his or her class?

The question speaks for itself, but here are a few examples:

  • My cleric’s holy symbol is broken, but she still has an arcane orb.
  • My bard wants to sing and speak instead of using instruments, and would like to use a simple amulet (rather than an instrument) for the material components.
  • My wizard has a yew wand (a wand nonetheless, but druidic) and would like to use it to cast arcane spells.
  • My warlock wants to use a quarterstaff instead of a standard staff as an arcane focus.

Are any of these cases possible? I think this might be a RAI issue, but if there are any hard references in the rules, they will be prioritized.

How to find out what spells would be useless to a blind NPC spellcaster?

This question: What are the drawbacks of a blind spellcaster? doesn’t really touch on all the spells that are off-limits to a blind spellcaster.

As established in this question (Do you need line of sight to cast spells on someone?), you do not need to see the target unless it is a spell that states “that you can see”.

I want to make a list of all such spells, but the only comprehensive online resource I know of, dndbeyond, has no “filter by spell text” option. What is a way to find out, short of reading through every spell in every book I own, what spell options are off-limits?

I’m not looking for ways to circumvent the blindness, such as magic items or spells like darkvision (whether or not that works).

Can a spellcaster choose to deal nonlethal damage with a touch weapon-like spell?

The rules for nonlethal damage state:

You can use a melee weapon that deals lethal damage to deal nonlethal damage instead, but you take a –4 penalty on your attack roll.

Does this allow for a spellcaster wielding a weapon-like touch spell to deal nonlethal damage with that spell’s effect without needing to take a metamagic nonlethal substitution feat?

Relates to this question, but this specific question seems to be unanswered.

Prometheum Exxet: Do any of the Spellcaster item powers grant the item Zeon regen?

My GM and I have a definite disagreement over the text of the Stored Energy modifier in particular – the text in English is as follows.

If the object is itself has stored energy (via Zeon Container), the user may use its contained energy to cast the spells it contains. The transfer rate is 50 Zeon points per Combat Turn. The item gathers energy as if it was a spellcaster with Magic Accumulation 50. (Prometheum Exxet, 28, emphasis mine.)

Is his reading, that the item does not have Zeon regeneration just because it has magic accumulation, correct, or is my reading, that “as a spellcaster” includes ZR, MA, and…such, correct? Additionally, do any other modifiers seem to give an item ZR of its own? (I ask because I want to create items that work by themselves, and having no ZR on any of them quite handily precludes that save for reaching to the ‘GM fiat’ table.)

Does a spellcaster need to be willing for a Mezlan to use its Store Spells (Su) ability?

Mezlan is a type of morphic ooze with a special ability called Store Spells.

A mezlan can store up to 10 levels of spells that it can then cast freely. The spells imbued into a mezlan are each activated with the same casting time as the original spell, and are cast at the caster level of the spellcaster who provided the spells. A mezlan can store and use arcane, divine, and psychic spells, and can carry spells from any number of casters at a given time. To fuel this ability, a caster must cast her spells into the mezlan as if storing a spell in an item like a ring of spell storing. The mezlan must be aware of the incoming spell, choose to lower its spell resistance, and ready an action to absorb it. Only spells that have the mezlan as a target can be absorbed in this way; area spells can’t be absorbed. If a mezlan already holds 10 levels of spells, it cannot absorb more spells, and any spell of a level exceeding a mezlan’s available spell level is not absorbed and affects the mezlan normally. Once a spell is absorbed in this manner, the mezlan can use the spell as its own.

I can’t tell if both parties must be willing or if the Mezlan could use Spellcraft to identify an incoming spell (after dropping it’s SR) and Store it when a spellcaster is unaware of its abilities.