Aberrant Terrain.  The ground within 20 feet of the Old Man is corroded and partially dissolved by its corrosive nature.  This area is considered difficult terrain.

Spider Climb.  The Old Man can climb vertical surfaces, including upside down on ceilings, without needing to make an ability check

Dark Body.  The Old Man has advantage on Dexterity (Stealth) checks made in dim light or darkness.

Object Stride.  The Old Man can enter a surface (such as a wall, floor, or ceiling) and emerge in another surface within 30 feet by spending 5 feet of movement.  The surfaces entered and exited must be connected, and the 30 feet must be measured along the surface.  The Old Man doesn't provoke attacks of opportunity while moving in this way.

Corrosive Aura.  A creature that touches the Old Man or hits it with a melee attack takes 5 (2d4) acid damage.  This damage is included in the Old Mans own attacks.

Aberrant Nature.  The Old Man does not require food, water, air, or sleep and is immune to the effects of exhaustion.

Multiattack. The Old Man makes two touch attacks.

Touch. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 26 (6d6 + 5) necrotic damage plus 5 (2d4) acid damage, and the target is grappled (Escape DC 18).  The old man cannot grapple more than 2 creatures at a time.  While grappling a creature, one of the Old Man's touch attacks must target that creature.

Acid Splash. Ranged Weapon Attack:  +9 to hit, range 30/60 ft., one target.  Hit:  19 (4d6 + 5) acid damage.

Hurl Through The Void (2/day).  The old man makes a touch attack against a creature it is grappling.  On a hit, apply damage as usual, then the creature is transported into a demiplane of the Old Mans own horrendous design.  While in the demiplane, creatures are [condition]paralyzed/condition].  At the end of the Old Man's next turn, a transported creature returns to the space it previously occupied, or the nearest unoccupied space, and takes 33 (6d10) psychic damage and 33 (6d10) necrotic damage from the experience.

The Old Man can take 3 legendary actions, choosing from the options below.  Only one legendary action can be used at a time and only at the end of another creatures turn.  Able regains spent legendary actions at the start of his turn.

Acid Splash.  The Old Man uses it Acid Splash attack.

Object Stride.  The Old Man uses its object stride ability once.