An Cú Dubh

Keen hearing and smell. The Black Hound has advantage on Wisdom (perception) checks that rely on hearing or smell.

Multi-attack. The Black Hound makes two attacks, both of which have to be claw attacks.

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

HP Pools (50/50/50). The Black hound has three HP pools. When the threshold is reached for a pool, it immediately uses it's reaction to become invulnerably and trigger the corresponding effect.

If the Black Hound is reduced to 100 HP, it's form begins to shimmer and displace. It casts Mirror Image on itself.

If the Black Hound is reduced to 50HP, skeletal wings erupt from its back. Each wing has a set of primary, secondary, and tertial spinal bones. Whenever an attack would hit the hound, it can cause one set to shatter instead, negating the attack. A critical hit causes to sets to shatter. So long as the Hound has at least one set remaining, it has a 20ft. fly speed.

Claw. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 4) [slashing] damage. If the target is a creature, it must succeed on a DC 15 Strength saving throw or be knocked prone.

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 4) [piercing] damage.

Frightful Presence. Each create of the Hound's choice that is within 120 ft. of it and is aware of its presence must succeed on a DC 14 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 Hound's Frightful Presence for the next 24 hours.

Misty Step. The Hound teleports up to 30 ft. to an unoccupied space it can see.

Ill-Luck. When The Black Hound is attacked by a creature within 30 feet of it that it can see, it can use its reaction to impose disadvantage on the attack roll, causing coiling darkness to spray before the attacker before it hits or misses. An attacker that can’t be blinded is immune to this feature.

The Hound can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The Hound can not use the same legendary action again until all other options have been used. The Hound regains spent legendary actions at the start of its turn.

Reality Warp (Costs 1 Action). The hound targets one creature and corrupts their core ideal. The effects of this action vary depending on the target.

Huck. Roll 1d10 and 1d20. The d10 determines the section of Ben's DM notes, and the d20 determines a section that is instantly disintegrated.

Iggy. Pull one card from the deck of Creation. On a successful DC 22 Arcana check, the creation is friendly towards the target. 

Randir. All creatures fighting the Black Hound must succeed on a DC 18 Charisma saving throw. On a failed save, targets swap places with Randir. If Randir is pulled in multiple directions, he takes 3d6 slashing damage.

Arlington. All creatures fighting the Black Hound must succeed on a DC 18 Dexterity saving throw. On a failed save, reroll initiative at disadvantage.