If you look closely, this thing doesn't actually speak Common. It speaks Undercommon, which is a rare language spoken by usually evil beings underground. However, Wizards of the Coast did include a telepathic Nothic in LMoP, and they are quite good at stealth (+6 bonus), so your DM wasn't pulling your leg. At the end of the day, DnD is a game of make-believe where the DM has full reign to change whatever they want, so it would completely be in their power to pull your leg if they wanted to :) 

Nothics are half cute half creepy-

Gained the loyalty of the the one under Tresendar Manor, his name is Jeremy and got Talon the sword and the lost deed to the manor from him, 10/10 am ready to make him my butler. 

Quote from VladTheBigFailure >>

Weird Insight is such a cool move, but I wonder how you'd use it when a Nothic is alone. What kind of secret could they use against a party?

 well, we were playing lost mine of phandelver and in the redbrand hideout, we disguised ourselves as redbrand members. So far the nothic remained friendly. However, if the nothic used weird insight, it would have discovered the secret. (and bad news for us! but it didn't. XD we got past)

Quote from Rashkavar >>

"They are fairly intelligent"

 When the Nothic is smarter than everyone in your party...

was it in mine of something

what are there behaver like? 

There is a member on my party that has, accidentally, caused a fire that killed it's father and the parents of other member. They do not know it quite, but this members knows he caused three deaths on his rage (barbarian) and knows his father was one, and refuses that this was his fault. i'll use this way. There are an another member that killed the one who killed it's father, but the death of his father was also her fault, and she lies to herself of it, blaming the one who killed him in the first place

This enemy is super interesting, it's a shame more people don't know about it.

I created an undead "Gothic Nothic" for my homebrew. One ability is its gaze causes a turn to stone effect, even when beheaded, (like a Medusa). It's on a Barbarian's shield right now. It's good for a laugh during combat.

Quote from RaifTwelveKill >>

Vax... You killed an NPC

AMAZING Mission complete! THAT right there is why you're the best Boss. The one and only!"

Being a D.M. playing as a Nothic is really fun, as long as the players don't chicken out because they have never faced anything more powerful than a Bugbear.

The Nothic was a surprise hit in my LMOP campaign. I used my characters flaws as ways to mock each of them. I had all of them nervous whispering all their insecurities to them.

Quote from TheDungeonDude >>

the first time I encountered tis thing I shit my pants

 Weekling. my party befriended it and turned it. Against the people in the people the lived in the place (we were trying to defeat them) and my brothers were obseced with it for the rest of the game but little do they know in my current campaign they are about to enter an area with bloodthirsty nothics

They could learn something like where the party is staying - then it could try and escape to turn into a bigger encounter later.

So ah I Have a magic city. I think I might use one or a group of these as librians in the deepest darkest parts of the library. Esiantally they run the restricted section of an academic library in a town wear almost anything goes. They keep the knowledge the most powerful mages in the world are afraid of.

These are also helpful in back stories of younger Wizards as a cautionary tale or boogeyman like tales for arcane practitioners who are far to eager to learn to much. Either told from parents, grandparents or just older mentors. 

My party managed to befriend the one in the redbrand hideout and they’re using it as a guard while they turn the hideout into an adventurers inn/guild thing. They also have a illusionist hobo they found in a dungeon that is going to be the innkeeper

Okay, so im just going to go ahead and start the controversy. Since Weird insight says uncharmable creatures are immune to its effects, that means Elves and Half Elves have Advantage on their Deception check to avoid it. As Fey Ancestry lets them have advantage on saves that cause the Charmed condition.

In addition, i would reccommend using the 2nd lvl spell Nondetection, which means you cannot be targeted by divination spells or seen through magical sensors, (while i still wouldnt rule that this makes you invisible to Nothics entirely, a creature under this spell should be immune to Weird Insight) it can be a bit pricey to continually cast though. 50gold worth of components each time.

Finally, the rare magic item Amulet of Proof against Detection and Location: it is attunement, but for a BBEG or a player who'se backstory is they are being hunted, this item is a godsend. It is a constant nondetection spell without the monetary cost and only requires an attunement slot.

Read a good bit of lore on these things. Apparently they were arcane casters who got a little too close to a secret that Vecna wanted to keep hush hush. 

I would also rule some of them being cursed, or willing servitors of Savras the unseeing. (Lesser diety of divination in Forgotten Realms)

Ah, but that is exactly it. Do you really expect the evil wizard turned Lich, turned God would WANT people to be able to do what he did?

If i were a maniacal overlord bent on bending the universe to my whims the last thing i would want is some upstart using the techniques that i spent millenia inventing from the ground up to pull the rug out from under me and take the universe himself.

The reasonable solution? Curse the everloving crud out of them and eliminate their memories, including the memory of the key secret they discovered.