Can a Pact of the Chain Imp familiar confer invisibility and flight onto its Warlock?

Question

An imp has an innate fly speed of 40 and a carrying capacity of 45lbs (15 X 6 STR / 2) . And, it can turn invisible at will, conferring that to any equipment it is wearing or carrying:

The imp magically turns Invisible until it attacks, or until its Concentration ends (as if concentrating on a spell). Any Equipment the imp wears or carries is Invisible with it.

There are 3 immediately relevant considerations:

1. Magic items

• a.Bag of holding - Does not weigh more than 15 lbs.

The bag can hold up to 500 pounds, not exceeding a volume of 64 cubic feet. The bag weighs 15 pounds, regardless of its contents.

A PC can enter the bag, the imp can carry it to confer invisibility. The bag does run out of oxygen in 10 minutes:

Breathing creatures inside the bag can survive up to a number of minutes equal to 10 divided by the number of creatures (minimum 1 minute), after which time they begin to suffocate.

But, there is nothing to stop the player from keeping his head and neck outside of the bag. The bag's oxygen will deplete in that time, but 10 minutes of flight and invisibility is more than enough to infiltrate/ escape a dire situation/ achieve a strategic position. Especially if you retreat back into the bag ("lather; rinse; repeat") when enemies get close.

• b. Sack of Carrying

This item appears like a normal, large sack that weighs 5 pounds. It can hold 12 cubic feet or 360 pounds, without changing its total weight. Creatures in the sack can breathe normally. You can put objects or creatures into the sack or take them out with a normal object interaction.

2. Enlarge/Reduce - Reduce reduces a creatures weight by a factor of 8.

The target's size is halved in all dimensions, and its weight is reduced to one-eighth of normal.

For many humanoids (<= 360lbs), this is well within an imp's carrying capacity. So, for a second level spell, a PC gets a full minute of flight AND invisibility.

Moreover, RAW this invisibility does not end if the PC attacks or casts a spell, because the imp is the one concentrating on the invisibility, not the PC. So, theoretically, from 120 ft in the air, totally invisible, the PC can Eldrich Blast to his or her heart's content.

There is a nuance (mentioned here: Imp familiars Invisibility affecting a PC) that a PC is not strictly "Equipment", and therefore does not receive invisibility. But, this is trivial to work around. For example,

A basket can hold 2 cubic feet/40 pounds of gear.

A Reduced medium PC could sit (perhaps awkwardly) in the basket, surrounded by a cloak and receive the same benefits of being inside a Sack or Bag of Holding.

3. Any creature weighing < 40lbs. - ibid.

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Is it RAW legal? Isn't this grossly overpowered?

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Note on carrying capacity:

Lifting and Carrying

Your carrying Capacity is your Strength score multiplied by 15. (...) For a Tiny creature, halve these weights.

Answer 1

This is unlikely to work

The primary problem with this plan is not that a carried creature is not affected by invisibility¹. The problem is rather that the premise "there is nothing to stop the player from keeping his head and neck outside of the bag" is not correct.

Bag of Holding says:

Retrieving an item from the bag requires an action.

As further supported by this question about breathing from an open bag of holding, the opening of the bag is not a normal opening. You cannot shake out its contents with an object interaction, for example: retrieving an item from the bag explicitly requires an action.

This means if you are in the bag, you cannot just stick your head out. You need someone outside the bag to retrieve you with an action, and you also cannot be half-way in or out the bag. You either are in the bag, or you are retrieved and outside the bag after being retrieved.

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¹ In this case, the wording for an imp's invisibility extends only to equipment, while the question you link and the invisibility spell uses a wording of "anything worn or carried", so I think invisibility would not apply here, either.

One can argue that a carried creature should count as equipment, but the voting consensus on this points to "no".

Putting the PC into a sack or basket will not reduce their weight, so the imp will not be able to carry them. Using reduce to shrink them in this case might work around that (don't forget the weight of their equipment), but they still would not be invisible: probably not to begin with, and for sure not anymore once they came out of cover to have line of effect for their casting spells.

Putting them into a bag of holding makes the invisibility question a moot point, because in that case, the only thing the imp carries and makes invisible is the bag; the PC is not even there to be made invisible, they are then in an extradimensional space.

Answer 2

RAW, a PC can gain some of these benefits.

The apparent consensus for the respective scenarios:

1. Inside a magic item.

• a. An imp CAN carry a PC inside a Bag of Holding, which provides many of the practical benefits of both flight and invisibility. But, the PC is probably limited by being within the extradimensional space of the bag.

• b. An imp CAN carry a PC inside a Sack of Carrying, which provides many of the practical benefits of flight, and the question of invisibility is left up to the DM.

2. An imp CAN carry a PC, weighing less than 45lbs. So, a 2nd level Reduce spell can effectively grant a medium creature (weighing 360lbs = 45lbs * 8 Reduced weight) 1 minute of flight by which to avoid traps or obstacles. But, this does NOT necessarily confer invisibility to the PC.

Reduce:

Duration: Concentration, Up to 1 minute (...) The target's size is halved in all dimensions, and its weight is reduced to one-eighth of normal.

3. Ibid

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Imp's carrying capacity with 6 STR = 45lbs:

Your carrying Capacity is your Strength score multiplied by 15. (...) For a Tiny creature, halve these weights.