Chasmfiend

Dark Lurker. The Chasmfiend can blend seamlessly into darkness. While in dim light or darkness, it can take the Hide action as a bonus action and has advantage on Dexterity (Stealth) checks. While hidden in this way, it is considered invisible to creatures without darkvision or truesight.

Tunnel Ambusher. The Chasmfiend is adapted to fighting in enclosed spaces. It can move through passages as narrow as 10 feet wide without squeezing and can climb along walls and ceilings as easily as floors. When it moves through a tunnel, it leaves no trace of its passage and has advantage on attack rolls against creatures that have not yet taken a turn in combat.

Aura of Dread. When a creature starts its turn within 30 feet of the Chasmfiend and the Chasmfiend is in darkness, it must succeed on a DC 18 Wisdom saving throw or be frightened until the end of its next turn. If the creature's saving throw is successful, it is immune to the Aura of Dread for the next 24 hours.

Shadow Regeneration. The Chasmfiend regains 20 hit points at the start of its turn if it is in dim light or darkness. If it takes radiant damage, this trait doesn't function at the start of the Chasmfiend's next turn.

Legendary Resistance (3/Day). If the Chasmfiend fails a saving throw, it can choose to succeed instead.

Multiattack. The Chasmfiend makes three attacks: one with its Bite and two with its Tentacles.

Bite. Melee Weapon Attack: +15 to hit, reach 10 ft., one target.
Hit: 30 (4d10 + 9) piercing damage. If the target is a creature, it is grappled (escape DC 20). Until this grapple ends, the target is restrained, and the Chasmfiend can't bite another target.

Tentacle. Melee Weapon Attack: +15 to hit, reach 30 ft., one target.
Hit: 22 (3d8 + 9) bludgeoning damage, and the target is grappled (escape DC 20). The Chasmfiend has six tentacles, each of which can grapple one target. If a creature starts its turn grappled by a tentacle, it takes 14 (3d6 + 4) bludgeoning damage.

Horrifying Drag. If the Chasmfiend has a creature grappled, it can use a bonus action to move up to 30 feet while dragging the creature with it. If the creature is dragged into darkness, it must succeed on a DC 18 Wisdom saving throw or be paralyzed until the start of its next turn.

Tentacle Slam (Recharge 5–6). The Chasmfiend slams all creatures it has grappled into the ground or another surface. Each creature grappled by it must succeed on a DC 20 Strength saving throw or take 45 (10d6 + 9) bludgeoning damage and be stunned until the end of its next turn. On a success, a creature takes half damage and is not stunned.

The Abyssal Chasmfiend can take 3 legendary actions, choosing from the options below. It can only take one legendary action at a time and only at the end of another creature's turn. It regains spent legendary actions at the start of its turn.

Tentacle Attack. The Chasmfiend makes one tentacle attack.

Disappear into Darkness (Costs 2 Actions). The Chasmfiend moves up to its speed into an area of darkness or dim light and can take the Hide action.

Eerie Whispers (Costs 3 Actions). The Chasmfiend emits a deep, resonating sound that echoes through the tunnels, causing fear to creep into the minds of its prey. Each creature within 60 feet that can hear it must succeed on a DC 18 Wisdom saving throw or be frightened for 1 minute. A frightened creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.