Light Sensitivity.  While in bright light, the abomination has disadvantage on attack rolls, as well as on Wisdom (Perception) checks that rely on sight.

Change Anatomy.  As a bonus action, the abomination can change between its bipedal and quadrupedal forms.  In its bipedal form, the abomination's walking speed is 30, it has a climbing speed of 30, and it can take the grasp attack.  In its quadrupedal form, the abomination's walking speed is 45, it doesn't have a climbing speed, and it can take the bite attack.

Multiattack.  The abomination uses its Frightful Presence and makes one melee attack.

Grasp (Bipedal).  Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 25 (3d12 + 6) bludgeoning damage.  If the target is Medium or smaller, the target is grappled (escape DC 18).  Until the grapple ends, the target is also restrained, and takes 12 (1d12 + 6) bludgeoning damage at the start of their turn.  The abomination has two arms, and each of which can grapple a creature.

Bite (Quadrupedal).  Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 25 (3d12 + 6) piercing damage.  The target must make a DC 17 Constitution saving throw or become poisoned for one minute.  While poisoned, the creature gains 1 level of exhaustion.  At the end of each of the target's turns, it can repeat the saving throw, ending the poison and exhaustion level on a success.

Frightful Presence.  Each creature of the abomination's choice that is within 120 feet of it and aware of it must succeed on a DC 17 Wisdom saving throw or become frightened for 1 minute.  A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.  If a creature's saving throw is successful or the effect ends for it, the creature is immune to the abomination's Frightful Presence for the next 24 hours.