One of my players has a Warlock with the Pact of the Chain. This PC has a quasit familiar. If we are applying the rules correctly…
Pact of the Chain
You learn the find familiar spell and can cast it
as a ritual. The spell doesn’t count against your number of spells
known. When you cast the spell, you can choose one of the normal
forms for your familiar or one of the following special forms: imp,
pseudodragon, quasit, or sprite. […]Find Familiar
[…] While your familiar is within 100 feet of you, you can
communicate with it telepathically. Additionally, as an action, you
can see through your familiar’s eyes and hear what it hears until the
start of your next turn, gaining the benefits of any special senses
that the familiar has. During this time, you are deaf and blind with
regard to your own senses. As an action, you can temporarily dismiss
your familiar. […]VARIANT: QUASIT FAMILIAR […] Familiar. The quasit can serve
another creature as a familiar, forming a telepathic bond with its
willing master. While the two are bonded, the master can sense what
the quasit senses as long as they are within 1 mile of each other.
While the quasit is within 10 feet of its master, the master shares
the quasit's Magic Resistance trait. At any time and for any reason,
the quasit can end its service as a familiar, ending the telepathic
bond.
… then he can communicate with it within 1 mile and see what it is seeing.
For example, in a mission where PCs have to peek at the enemy camp, he could just stay hidden and send the quasit exploring the camp. The quasit can turn invisible and polymorph into a centipede, so it can go anywhere (even inside a building), and the Warlock can see everything.
Now, is this correct? It would seem really overpowered to me.
If it's correct, how can I limit this power in an acceptable way?
Best Answer
It's almost correct, but not quite.
There are two separate, unrelated ways to get a familiar in D&D 5e, and you're trying to combine them.
The first method is by use of the Find Familiar spell, which allows you to summon a celestial, fey, or fiendish spirit that takes the form of any of a list of creatures. This list is expanded by the warlock pact of the chain. This familiar is perfectly obedient, can be resummoned when it dies, can be hid in a pocket dimension, deliver touch spells that you cast, and everything else specified in the spell description. This method gives you a familiar with the basic stats of the chosen creature, not the "variant: familiar" traits of the chosen creature (unless your DM chooses to have that creature type appear).
The second method is by finding a quasit, imp, or pseudodragon that has the "variant: familiar" trait (which is 100% up to the DM), and enlisting it as a familiar by interacting with it. This familiar has only the traits listed in the stat block for that creature, including the variant traits, but none of the traits of familiars given by the Find Familiar spell. No pocket dimension, no touch spells, no limitations on what actions it can perform, and if it dies, it's just dead. This type of familiar is an NPC controlled by the DM (much like a hireling or other follower), and is only as obedient to the PC as the DM says it is, using the MM entry as a guide.
Now that we've established how things actually work, we can address your real concern. Your warlock can't yet communicate at a great distance, but he can soon. There is a warlock invocation available to him called Voice of the Chain Master that does the same thing, but with unlimited range on the same plane. As you've realized, this ability has some incredible potential, especially for scouting.
It's not any more powerful than other options and should not be limited.
Let's compare it to some other invocations. There's one that lets a warlock cast Disguise Self as at will. This would let him see a guard, and appear exactly like that guard and just walk around the enemy camp unimpeded. Or maybe impersonate the leader of the camp and just take it over without even a struggle. There's another invocation that lets the warlock cast arcane eye at will, which gives you a way better scout than an easily killed creature. A familiar, even an invisible one, still has to succeed on a Dexterity (stealth) check to avoid being heard and then easily killed. An arcane eye does not.
So, in order to fully utilize this scouting ability, your warlock has to pick the chain pact and spend one of his few, precious invocations, both of which are huge opportunity costs. He deserves something in return. This something is you not limiting it. It already has a flaw in still being able to be killed by anything that hears it, or smells it. That's right, just about any pet wolf is going to be enough to catch this familiar. It doesn't need any more limitations.