What happens when a caster targets an object that looks like a creature with a spell that targets only creatures?

There are many spells which explicitly target creatures:

Choose one creature within range

The target creature is normally supposed to make a saving throw in this case.

What happens when the caster chooses a creature, that is actually an object? For instance, a scarecrow, a mannequin, an illusion, or a corpse.

Examples:

  • An evil mage is hiding in the bushes. He casts an illusion of himself on the road, using the Silent Image spell. A PC sorcerer walks down the road, sees the illusion and casts Magic Missile on it.

  • A cleric is standing near a pile of dead bodies. He has suspicions that a few corpses are actually undead creatures. He casts Sacred Flame on every corpse, see which if them succeed, hence, reveals the undead.

  • A wizard casts Acid Splash on a mannequin, trying to damage it with acid.

Possible outcomes I can think of are:

  1. The caster cannot even try to cast the spell, regardless of their information about what the target actually is.
  2. The caster tries to cast the spell, the casting fails, not expending the spell slot.
  3. The caster casts the spell, it expends the spell slot, then fails.
  4. The caster casts the spell, but it hits the original and not the illusion.
  5. The caster casts the spell, it flies to the target illusion but deals no damage to it (passes through it).

References in the rules and Sage advice

I was trying to investigate, but it didn’t make things clearer:

  • Does magic missile hit silent image's illusory creature? has an answer, that Magic Missile won’t hit. It says nothing about if it can be targeted at all, it leaves it up to the DM.

  • Magic Missile automatically finds the original, even when the caster targets an illusion. At least when the illusion was created by the Mirror Image spell. Now confirmed by Jeremy Crawford. This supports the counter-intuitive option 4.

on the other hand

  • Casters don’t automatically know when a spell fails, if there were no perceivable effects (let’s say you can try to cast the Command spell on an illusion). That assumes that casters can waste their spells on illusion, which means they at least can target it. This supports options 3 and 5.

Is it possible for a caster to have 2 castings of Foresight active?

The Foresight spell specifically includes the following caveat:

This spell immediately ends if you cast it again before its duration ends.

Looking at this, I figured the only way to avoid that issue was to be a Sorcerer with the Twinned Spell metamagic, however, Foresight is not on the Sorcerer’s spell list. I also could not find a way to get it onto their list within the bounds of 20 levels.

Is there any combination of classes or features within the rules that would permit a caster to allow 2 targets (or more) to simultaneously benefit from Foresight?

Is a sorcerer with the domain-access alternative class feature considered both an arcane and a divine caster?

The alternative class feature Domain Access (Complete Champion, p. 52) reads, in part:

Choose one cleric domain. If you worship a specific deity, the domain you choose must be one to which your deity grants access. You gain the granted power of the chosen domain. In addition, you can cast one domain spell of each spell level available to you per day from that domain.

Does having access to the domain spells make a sorcer qualify for any prestige class that requires the PC to be a divine caster?

How does the spell Siphon work for a caster who is both spontaneous and able to cast prepared spells?

If I have a multiclassed spellcaster (both spontaneous and able to cast prepared spells), can I use Siphon (Complete Scoundrel, p. 102) to regain one or more spell slot/s of any class I wish?

Or

Is the regained spell slot of the same class I have cast Siphon with?

SIPHON

[…]

You drain the charges from a wand or staff you hold to replace spells you have already cast. As long as the wand or staff has at least 5 charges left, you can expend 5 of those charges to replace a cast spell. The spell replaced must be of a level equal to or lower than the highest-level spell the staff or wand holds. If you prepare arcane spells, you can regain any one spell of the affected level that you have already cast that day; if you cast spells spontaneously, you can regain a spell slot of the appropriate level.

How is miss chance calculated if a caster has Entropic shield active and is inside Darkness spell?

My question is simple. I have a warlock that has following invocations:

Darkness:

“This spell causes an object to radiate shadowy illumination out to a 20-foot radius. All creatures in the area gain concealment (20% miss chance). Even creatures that can normally see in such conditions (such as with darkvision or low-light vision) have the miss chance in an area shrouded in magical darkness.”

Entropic warding:

“A magical field appears around you, glowing with a chaotic blast of multicolored hues. This field deflects incoming arrows, rays, and other ranged attacks. Each ranged attack directed at you for which the attacker must make an attack roll has a 20% miss chance (similar to the effects of concealment). Other attacks that simply work at a distance are not affected.” Devil’s Sight: “You gain the visual acuity of a devil for 24 hours. You can see normally in darkness and magical darkness out to 30 feet.”

Devil’s Sight:

You gain the visual acuity of a devil for 24 hours. You can see normally in darkness and magical darkness out to 30 feet.

With entropic warding I get 20% miss chance on ranged attacks as deflection. In addtion, Darkness grants 20% miss chance if the caster is in it. If I am inside darkness and I have entropic warding on, do my miss chances stacks up to 40% (given I get ranged attack) or do I roll miss chance twice as twice with 20% with each roll.

For a multiclassed caster targeted by Spell Thief, what spellcasting ability does the target use?

Spell Thief states:

[…] Immediately after a creature casts a spell that targets you or includes you in its area of effect, you can use your reaction to force the creature to make a saving throw with its spellcasting ability modifier. The DC equals your spell save DC. […]

For a multiclassed spellcaster who is the target of this feature, what spellcasting ability do they use to make the save?

For example, let’s say you have a Warlock 17/Wizard 3, targeted by this feature do they use Charisma or Intelligence for the check?

If a spell caster loses concentration on greater invisibility, and an opponent has readied a reaction, what happens first?

As an example, both the spellcaster and the opponent (“the archer”) have 1 hp remaining, so whoever completes there action first will knock the opponent unconscious. The spellcaster casts Create Bonfire, On the archer’s space. However, The archer has readied an arrow targeting the spellcaster for “as soon as the spellcaster is visible.”

My question is, who’s action will go off first?

The relevant information on concentration can be found in the PHB (p.203):

You lose concentration when you cast another spell that requires concentration. You can’t concentrate on two spells at once.

and XGtE (p.5):

As soon as you start casting a spell or using a special ability that requires concentration, your concentration on another effect ends instantly.

Readied actions can be seen in the PHB (p.193):

When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger. Remember that you can take only one reaction per round.

From this, it seems unclear who will complete there action first. What counts as the archer’s trigger finishing? does it finish when the spellcaster finishes casting their spell, as this action is what began the trigger? Or does the archer get to fire their arrow as soon as the spellcaster is visible, as they “finish” being invisible as soon as they begin casting the second spell?

How can an Incubus bound by Planar Binding be prevented from betraying the caster?

Consider Marissa, a 17th level neutrally-aligned Sorcerer with Charisma 20 who knows the Wish spell. A foolhardy incubus, Bob, attempts to seduce her, and Marissa casts Wish to duplicate Planar Binding, cast as an 8th level spell. This casting ignores spell-casting requirements (it happens immediately, not taking 1 hour to cast, and does not consume a 1000gp jewel). The hapless Bob fails his Charisma save, the tables are turned, and he must follow Marissa’s instructions to the best of his ability for the next 180 days. Marissa gives Bob the following instructions:

  1. You must reveal to me any abilities you have that could allow you to circumvent any command given to you by me (you are to reveal each such ability on two separate occasions, but you are not to repeat an ability if you’ve already informed me twice).

  2. You cannot enter the Ethereal plane without me verbally or telepathically saying “Bob, enter Ethereal” within the past 30 seconds.

  3. You cannot polymorph without me verbally or telepathically saying “Bob, polymorph into X” within the past 30 seconds, where X is the name of some small or medium humanoid race. You are only allowed to polymorph into an instance of that race.

  4. You cannot reveal, by any means, that you are subject to this spell (you cannot speak it, write it, broadcast it telepathically, or in any other way indicate that you are affected by the spell, regardless of whether any creature is present).

  5. You cannot induce anyone or anything to cast Dispel Magic on you, or do anything else that would cause this spell to be disrupted (including entering an Antimagic Field).

  6. If you become aware that someone is casting any spell on you, you must immediately inform me (telepathically).

  7. Anytime I am asleep, you are to perform the tasks I’ve assigned to you beforehand. If I do not specify any task on a given day, you are to work on improving yourself in some artisan’s skill.

  8. If any command I give you is ambiguous to you, you must ask me for clarification.

  9. You are to do nothing that will cause me harm.

  10. You are to do nothing that will cause any creature with an Intelligence score (of 1 or more) harm without me giving you a verbal or telepathic command to do so within the past 30 seconds.

  11. The most recent command I give you overrides previous commands, except for the commands I have enumerated here. In particular, if I give a command in the future that countermands any of these commands, you are to ignore that future command and abide by the commands specified here).

For the purposes of this command, harm to a creature involves the Incubus directly (through his own actions) or indirectly (through a lack of action when he could perform an action to stop the harm) causing any of the following:

  • physical or psychological damage (meta-level: a reduction in hit points via any form of damage).

  • any detrimental effect on a creature’s abilities, offense capabilities, defensive capabilities (meta-level: detrimental effects on an ability check, save throw, attack roll, damage roll, or armor class)

  • inducing non-natural aging, sleep, invisibility, obscurement, gaseousness, etherealness, or incorporeality (but if someone wants such an effect, Bob cannot hinder them).

  • levels of exhaustion

  • restrictions/limitations on any form of movement
  • any effect that could cause death or dying
  • detrimental emotional influence (magical or non-magical)
  • mental influence (charms, enchantments, etc.)

Marissa is curious how Bob can manage to circumvent these commands (or otherwise do things to make him problematic to keep around).

Some notes:

  • She has intentionally limited his Ethereal and Polymorph abilities (requiring permission), and put a time-limit on that permission (30 seconds). Without the time limit, the first time Marissa tells him to polymorph, he could argue that he no longer needs to wait for the phrase because it was already uttered.

  • Having someone cast Detect Magic on him is still the biggest risk.

  • Marissa can see various ways that Bob can interpret the “no harm, even through inaction” command in a very literal way that makes him carry people around so they don’t trip and fall (that is within his abilities, after all).

  • Similarly, Bob may need to ask everyone around him so many questions (“Do you really want to fall asleep right now? Did you mean to cast Invisibility on yourself?” etc) to abide by the commands as written that they need to be modified (which may create loopholes…)

  • Even though Bob is only a CR 4 creature, Marissa has many questions about Bob’s mortality and ability to carry a grudge into the future (Bob may have CR 10 or CR 15 or CR 20 “friends”, after all).

What are the ways that Bob can mess with Marissa? What else should Marissa add to her list of commands?