The Insanity Mod has lots to offer such as

no_moon.jar

include jumpscares at random moments, can crash and control your game,

25 new entities

and a cool but difficult bossfight

Boy and the Bath

this horror mod is based of an analog horror series called Doctor Nowhere

this mod makes to make minecraft scarier and scarier

using a wide variety of events

A guy called Guilt aims to play with you but its real goal is to torture you

DISTURBED

a shapeshifting nightmare is out there to find you, with an IQ that no one has ever seen so high

two of its forms Chase and Decay can be just killed normaly

however its other forms must be killed by locating their heart and destroying it

Herobrine's Return

Herobrine is the most powerful entity who scares anyone

this mod adds excactly that

a new biome called The herobrine biome has been added;

a wasteland full of trees with no leaves

he watches you from the distance and he uses his infected cows to haunt you

herobrine can use the chat to speak to you then suddenly cause a jumpscare and summon lightning

The Mimicer

Not much description on this'

The mimicer hides in dark corners and mainly spawns in caves

when going mining just watch your back.

The One Who Watches

This mod aims to make the player feel incredibly uneasy, fearful, and hyper observant.

Stay as far away from it as possible

He will find you and torture you much rather than killing you

Listen for footsteps, and always check behind you

the only way to stop himis to avoid him

The Origin

This entity will make noises and will mess with your head

He will hunt you in the caves non-stop

He always know where you will go and stalk you

insanity.jar