Sciaphobia.                       Any creature that can see it must either fail a DC 13 intelligence save or succeed on a DC 19 wisdom save (victim's choice), or become frightened of it. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success (wisdom) or fail (intelligence), and becoming immune to this trait for the next hour. 
This trait ignores immunities to being frightened that do not come from lacking a true mind.

Lygophobia. The area 20 ft. around the "Scotophobin" is always in dim light, sources of light within this area are extinguished upon entering it, and spells or other magical effects that attempt to brighten the area do not function.

Bangungut. The creature cannot sleep.

Without witness, without reward. It is unclear if a "Scotophobin" truly dies when reduced to 0 hit-points, or if death is at all really a concept to it. In any case, it leaves nothing behind, simply vanishing without a trace when defeated. 

Phantasmal. It can move through gaps as narrow as 1 inch wide without squeezing. 

Contingent Regeneration. If it starts its turn with less than 100 hit-points, the monster will regain 20 hp, unless on its previous turn it took damage of the type(s) fire or radiant from a magical source, or damage from a +2 magical weapon.

Chronophobia. The universe itself seems to hesitate during the monster's turn. It does not provoke opportunity attacks, and it ignores the effects of time stop.

Incomprehensible geometry. The space in a 10 ft. radius around the "Scotophobin" is difficult terrain for non-aberrations, as the thing's presence warps the space into impossible angles, additionally causing the following effects: 

• A non-aberration creature that starts its turn in the area or enters it for the first time on a turn must make a DC 19 wisdom saving throw or take (1d10) psychic damage and be poisoned until beginning of its next turn. 
• Ranged attacks made by non-aberrations that pass through the area are made at disadvantage.

This trait overrides preexisting difficult terrain in the area.

 Vertigo. The "Scotophobin" can walk upon any surface, including walls and upside-down on ceilings, as if on normal ground, as all gravity is objective-directional for the monster to any surface of its choice that it is touching. 
Because of this, it is unaffected by such things as reverse gravity, or any magical effect that would attempt to telekinetically shove it. Additionally, any creature that finds itself within 5 ft. of the "Scotophobin", will also experience gravity in the same way as it. 

 Ruin Mind. 
The "Scotophobin" chooses up to 3 creatures within 15 ft. of it. 
.....+10 to hit, Hit: target looses 12 (3d6+2) hit points from spontaneous organ-failure, and has its wisdom score reduced by 1d3, to a minimum of 1. If the target's AC is cleared by 5 or more, it takes an additional 3 psychic damage and must make a DC 13 wisdom saving throw.
If this attack reduces the target bellow 10 wisdom, if the attack lands with a critical hit, or if the target fails the wisdom saving throw, the target suffers one of the additional effects bellow of the monster's choice.        ...the monster may not choose the same effect twice in a row, and a single creature may not be subject to more than one effect at a time:

• Amnesia. The target looses a chunk of its memory, reducing its proficiency bonus by 1 for the next minute. 
• Anoxia. If the target cast a spell since the end of the monster's previous turn, it looses the ability to cast that spell until it completes a long rest. 
• Apnea. The target forgets how to breathe. At the end of each of its turns, it may make a DC 19 wisdom saving throw, remembering how to breathe on a success. 
• Mania. On the target's next turn, it may make one melee attack as a bonus action, but its targets are selected at random from those it can reach with its movement. 
• Paranoia. The target becomes frightened of one of its allies until the end of its next turn. While frightened in this way, it cannot willingly accept assistance from that ally. 
• Prosopagnosia. The target becomes incapable of distinguishing friend from foe by sight, and cannot recognize a person by seeing them. For the next minute, whenever the target uses an attack, spell or other ability where it chooses a target, that target must be chosen at random from among the creatures it can see within range, unless it has been convinced and guided by voices that it trusts to target something specific.   
• Seizure. If the target casts a spell on its following turn, it must make a DC 19 constitution saving throw or fall prone and end its turn. 

 Ravage Cognition. As a reaction to a creature within 15 ft. of it hitting it with an attack, the "Scotophobin" uses its Ruin mind on the creature, targeting the attacker with all 3 attacks.

Psychic Storm. As a reaction to being reduced below either 115 or 121 hit points, the thing lets out a telepathic shriek. Each creature within 60 ft. that has a wisdom score of 1 is stunned until the end of its next turn.