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:
- 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.
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.
- 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.
- Any creature weighing < 40lbs. - ibid.
Is it RAW legal? Isn't this grossly overpowered?
Note on carrying capacity:
Your carrying Capacity is your Strength score multiplied by 15. (...) For a Tiny creature, halve these weights.