Fiendish Vigor (2024)

Quote from Tarod_ >>

If it helps, we have this in the Sage Advice Compendium:

What level is a spell if you cast it without a spell slot?

Such a spell is cast at its lowest possible level, which is the level that appears near the top of its description. Unless you have a special ability that says otherwise, the only way to increase the level of a spell is to expend a higher-level spell slot when you cast it. [...]

 Well, there is:

Level 1: Pact Magic

Through occult ceremony, you have formed a pact with a mysterious entity to gain magical powers. The entity is a voice in the shadows—its identity unclear—but its boon to you is concrete: the ability to cast spells.

Cantrips. You know two Warlock cantrips of your choice. Whenever you gain a Warlock level, you can replace one of your cantrips from this feature with another Warlock cantrip of your choice.

When you reach Warlock levels 4 and 10, you learn another Warlock cantrip of your choice, as shown in the Cantrips column of the Warlock Features table.

Spell Slots. The Warlock Features table shows how many spell slots you have to cast your Warlock spells of levels 1–5. The table also shows the level of those slots, all of which are the same level. You regain all expended Pact Magic spell slots when you finish a Short or Long Rest.

For example, when you’re a level 5 Warlock, you have two level 3 spell slots. To cast the level 1 spell Witch Bolt, you must spend one of those slots, and you cast it as a level 3 spell.

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This is a special feature: "For example, when you’re a level 5 Warlock, you have two level 3 spell slots. To cast the level 1 spell Witch Bolt, you must spend one of those slots, and you cast it as a level 3 spell." - you just don't expend the spell slot

Quote from Ravenal >>

 Well, there is:

 

Level 1: Pact Magic

Through occult ceremony, you have formed a pact with a mysterious entity to gain magical powers. The entity is a voice in the shadows—its identity unclear—but its boon to you is concrete: the ability to cast spells.

Cantrips. You know two Warlock cantrips of your choice. Whenever you gain a Warlock level, you can replace one of your cantrips from this feature with another Warlock cantrip of your choice.

When you reach Warlock levels 4 and 10, you learn another Warlock cantrip of your choice, as shown in the Cantrips column of the Warlock Features table.

Spell Slots. The Warlock Features table shows how many spell slots you have to cast your Warlock spells of levels 1–5. The table also shows the level of those slots, all of which are the same level. You regain all expended Pact Magic spell slots when you finish a Short or Long Rest.

For example, when you’re a level 5 Warlock, you have two level 3 spell slots. To cast the level 1 spell Witch Bolt, you must spend one of those slots, and you cast it as a level 3 spell.

---

This is a special feature: "For example, when you’re a level 5 Warlock, you have two level 3 spell slots. To cast the level 1 spell Witch Bolt, you must spend one of those slots, and you cast it as a level 3 spell." - you just don't expend the spell slot

That's nice, but it's a feature which is under the "Spell Slots" section; it applies specifically when a Warlock is casting a spell with a spell slot. It doesn't say all spells a Warlock casts -- regardless of source -- get upcast.

Quote from Ravenal >>

Quote from Tarod_ >>

If it helps, we have this in the Sage Advice Compendium:

What level is a spell if you cast it without a spell slot?

Such a spell is cast at its lowest possible level, which is the level that appears near the top of its description. Unless you have a special ability that says otherwise, the only way to increase the level of a spell is to expend a higher-level spell slot when you cast it. [...]

 Well, there is:

Level 1: Pact Magic

[...] Spell Slots. The Warlock Features table shows how many spell slots you have to cast your Warlock spells of levels 1–5. The table also shows the level of those slots, all of which are the same level. You regain all expended Pact Magic spell slots when you finish a Short or Long Rest.

For example, when you’re a level 5 Warlock, you have two level 3 spell slots. To cast the level 1 spell Witch Bolt, you must spend one of those slots, and you cast it as a level 3 spell.

---

This is a special feature: "For example, when you’re a level 5 Warlock, you have two level 3 spell slots. To cast the level 1 spell Witch Bolt, you must spend one of those slots, and you cast it as a level 3 spell." - you just don't expend the spell slot

(sorry for snipping your text, just for brevity)

Or... you can consider Eldritch Invocations as Special Abilities, so they follow the "Casting without Slots" rules:

Casting without Slots

There are several ways to cast a spell without expending a spell slot:

Cantrips. A cantrip is cast without a spell slot.

Rituals. Certain spells have the Ritual tag in the Casting Time entry. Such a spell can be cast following the normal rules for spellcasting, or it can be cast as a Ritual. The Ritual version of a spell takes 10 minutes longer to cast than normal, but it doesn’t expend a spell slot. To cast a spell as a Ritual, a spellcaster must have it prepared.

Special Abilities. Some characters and monsters have special abilities that allow them to cast specific spells without a spell slot. This casting is usually limited in another way, such as being able to cast the spell a limited number of times per day.

Magic Items.Spell Scrolls and some other magic items contain spells that can be cast without a spell slot. The description of such an item specifies how many times a spell can be cast from it.

Btw, the SAC I quoted was updated for 5.5e:

What level is a spell if you cast it without a spell slot?

Unless the text says otherwise, a spell cast without a spell slot is cast at its lowest possible level, which is the level that appears in its description.

An example of an exception to this is often found in monster stat blocks. A monster with the Spellcasting action may have spells listed with a “(level # version)” parenthetical. This means the spell is still cast without a spell slot, but its effects are equivalent to a version of the spell cast using a spell slot of the specified level.

Quote from Tarod_ >>

Btw, the SAC I quoted was updated for 5.5e:

What level is a spell if you cast it without a spell slot?

Unless the text says otherwise, a spell cast without a spell slot is cast at its lowest possible level, which is the level that appears in its description.

An example of an exception to this is often found in monster stat blocks. A monster with the Spellcasting action may have spells listed with a “(level # version)” parenthetical. This means the spell is still cast without a spell slot, but its effects are equivalent to a version of the spell cast using a spell slot of the specified level.

 

 
Several things here. First the invocation in question: 

Fiendish Vigor

Prerequisite: Level 2+ Warlock

You can cast False Life on yourself without expending a spell slot. When you cast the spell with this feature, you don’t roll the die for the Temporary Hit Points; you automatically get the highest number on the die.

"Unless the text says otherwise, a spell cast without a spell slot is cast at its lowest possible level, which is the level that appears in its description"

No level appears in its (the text) description. Unless they are referring to the spell description, it would indicate that the spell slots the warlock has as a feature of their pact magic would be the definition of "the lowest possible level". 

The invocations often have "additions" to the granted spells, like you can only cast invisibility without expending a spell slot if you are in dim light or darkness. And different from what other discussions have been about higher level castings of invisibility, the invocation states you can only cast it on yourself like that. Same with jump. 

Also, this is different from magic initiate:
Level 1 Spell. Choose a level 1 spell from the same list you selected for this feat’s cantrips. You always have that spell prepared. You can cast it once without a spell slot, and you regain the ability to cast it in that way when you finish a Long Rest. You can also cast the spell using any spell slots you have.

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I definitely think it's intended that the spell is supposed to be cast at the spells lowest possible level - but currently the text isn't clear about it. So considering pact magic again:
"If another Warlock feature gives you spells that you always have prepared, those spells don’t count against the number of spells you can prepare with this feature, but those spells otherwise count as Warlock spells for you."

So you are casting these spells as warlock spells, using your pact magic spell slots - but without then expending that spell slot. In most cases, with invocations, the higher casting effect is negated by "can only be cast on yourself" - but logically, from the current text, you have to be using your pact magic spell slots that you don't expend. Otherwise the invocations would have "you can also cast the spell using any spell slots you have" like magic initiate.

Quote from Ravenal >>

"Unless the text says otherwise, a spell cast without a spell slot is cast at its lowest possible level, which is the level that appears in its description"

No level appears in its (the text) description. Unless they are referring to the spell description, it would indicate that the spell slots the warlock has as a feature of their pact magic would be the definition of "the lowest possible level". 

It is referring to the spell description. Remember that this rule applies to all situations where you cast a spell without a spell slot, not just this specific Warlock invocation.

Quote from Ravenal >>

 

I definitely think it's intended that the spell is supposed to be cast at the spells lowest possible level - but currently the text isn't clear about it. So considering pact magic again:
"If another Warlock feature gives you spells that you always have prepared, those spells don’t count against the number of spells you can prepare with this feature, but those spells otherwise count as Warlock spells for you."

This rule is not relevant since the invocations do not give you spells that are always prepared. "Always prepared" is a specific term in D&D rules and it very intentionally isn't used in these invocations.

There is no support anywhere in the rules for the idea that you can cast a spell "using" a spell slot but without "expending" it. There is no distinction between those two things.

Quote from Ravenal >>

 Well, there is:

Level 1: Pact Magic

Through occult ceremony, you have formed a pact with a mysterious entity to gain magical powers. The entity is a voice in the shadows—its identity unclear—but its boon to you is concrete: the ability to cast spells.

Cantrips. You know two Warlock cantrips of your choice. Whenever you gain a Warlock level, you can replace one of your cantrips from this feature with another Warlock cantrip of your choice.

When you reach Warlock levels 4 and 10, you learn another Warlock cantrip of your choice, as shown in the Cantrips column of the Warlock Features table.

Spell Slots. The Warlock Features table shows how many spell slots you have to cast your Warlock spells of levels 1–5. The table also shows the level of those slots, all of which are the same level. You regain all expended Pact Magic spell slots when you finish a Short or Long Rest.

For example, when you’re a level 5 Warlock, you have two level 3 spell slots. To cast the level 1 spell Witch Bolt, you must spend one of those slots, and you cast it as a level 3 spell.

---

This is a special feature: "For example, when you’re a level 5 Warlock, you have two level 3 spell slots. To cast the level 1 spell Witch Bolt, you must spend one of those slots, and you cast it as a level 3 spell." - you just don't expend the spell slot

 Dude. It's an example. Of casting a spell with a spell slot. It explicitly says "you must spend one of these spell slots".

To argue that you also get to upcast non-slot spells requires just a bit more textual support than you have presented.