I came to this comment section in hopes of finding exactly a comment like this. Thank you XD

This killed my grandma when she played dnd for the first time.

I’ve  have just encountered a nothic and one of my friends may or may not get brainwashed by weird insight. How should we fight it?

Out of all the names, why.

It doesn't appear to have any environmental tags

Well, we just had a campaign where our party's attempt to trick a Nothic failed because it learned our intent through Weird Insight. It's always dependent on how the encounter plays out.

@ Nsio_Zamek.. as a huge fan of disney and moster's inc/university were especially favorites of mine, i just laughed Way to Hard at this comment :-P 

Are these things telepaths, it doesn't look it to me other than the one spell? Our DM insisted he was following the guide in mines of phandelver but he had this talking in our heads so we couldn't fond it and it took us about a dozen perception rolls to be able to see it once even though we all had dark vision. He also made us roll a 20 perception to find a chest in the crevasse, is it really set that high on a starter campaign or was he messing us around? 

I'm going to use these as what happens to Great Old One warlocks if they slip up and draw the attention of their patron

Our Party found one of these in our game. My character is bordering on evil after spending time with a ruffian that the DM named Jeremy. Jeremy was taken off my wizard Pablo. Not being happy with this decision Pablo has taken the Nothic under his wing for 3 days. On the third day it has permission to eat and kill Jeremy. These events are still in play and will update when done.*Update Pablo died.

I'm imagining a corrupt magocracy that trains/employs these as a secret police...

Would probably make good scouts/infiltrators for an army that can gain their service too.

I once had a party who befriended one of these guys, it was all fun and games until they convinced it to help them clean out a bandit den and it started eating people. 

Quote from A_Random_Bard >>

I thought this was an Underdark monster, but I was making an encounter with one, and it didn't appear in the Underdark monsters section.

 RAW, the only language it can actually speak is Undercommon, which implies to me that they live in and/or around the Underdark as a rule of thumb. If they don't, then they would certainly know how to get there. 

 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...

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.