Dark Revelry: At night, while in the Grinning forest,the Yck-Grin revels in the darkness and gains advantage on all strength and stealth rolls.

Quit Playing With Your Food: If the Yck-Grin is standing still at night and adventurers manage to roll high enough to see it they will only see a large wicked looking grin in the darkness. The player or players that see this must make a wisdom saving throw (DC 17) or must become paralyzed for 1 minute. This only applies to the first player to spot the creature. If more than one player manages to see it then the one with the higher roll is affected. If two players have the same roll then they must both make the roll. 

What a Lovely Smile You Have: Players that see the Yck-Grin for the first time must succeed a wisdom saving throw (DC 15) or become frightened of the Yck-Grin for 1 minute.

Acid Rain: If the Yck-Grin is under 1/4 of its health, its acidic blood falls about it. Any creature that makes a melee attack against it takes 2d6 acid damage.

Noxious Vomit: The Yck-Grin vomits up glowing green acid mixed with small spikes that are leftovers from their previous victims in a 25 ft cone (DC 15 dexterity saving throw). Any creature hit takes 3d6 acid damage and 1d6 piercing damage. The ground where the vomit lands is then considered difficult terrain. And creature that moves through, starts, or ends their turn in the vomit takes 3d6 acid damage and 1d6 piercing damage. 

Umbral Sunder: Melee Weapon Attack: + 11 to hit, reach 10 ft., one or two targets. While Grappling a creature or with empty fists, the Yck-Grin can make one slam attack with each arm into the ground. The creatures take 11 (2d8+5) Bludgeoning damage for each slam. If the creature is slammed into the Noxious Vomit the damage from that applies (3d6 acid damage and 1d6 piercing damage). If the Yck-Grin uses both hands on a single creature the damage for the slam is increased to 22( 4d8+5) bludgeoning damage and can only be slammed once. 

Hungry Bite: Melee Weapon Attack: +10, reach 5 ft., one target. Hit: 14 (2d12 + 5)The Yck-Grin makes a bite attack at a creature. The creature must succeed on a DC 13 constitution saving throw or the Yck-Grin regains hit points equal to half the damage that was dealt to the creature.

Gotcha: if a creature trys to leave the Yck-Grin's melee range the Yck-Grin can choose, instead of making an attack of opportunity, to grapple a creature in one of its hands. If both of the Yck-Grin's hands are already holding a creature/player this reaction cannot take place. 

Grasping Shadow: The Yck-Grin can stretch its arms 60 ft and grasp a creature that it can see and can attempt to  grapple them. If the player fails a DC 18 strength or dexterity roll they are considered grappled and the Yck-Grin can pull them into melee range, while still holding them. The creature(s) can make escape attempts from the grapple any time they take damage or on their turn