Faultless Tracker. The retriever is given a quarry by its master. The quarry can be a specific creature or object the master is personally acquainted with, or it can be a general type of creature or object the master has seen before. The retriever knows the direction and distance to its quarry as long as the two of them are on the same plane of existence. The retriever can have only one such quarry at a time. The retriever also always knows the location of its master.

Innate Spellcasting. The retriever’s innate spellcasting ability is Wisdom (spell save DC 13, +5 to hit with spell attacks). The retriever can innately cast the following spells, requiring no material components.

3/day each: dimension door (can bring one incapacitated creature, which is considered willing for the spell), web, telekinesis

Multiattack. The retriever makes two foreleg attacks and uses its force or paralyzing beam once, if available.

Foreleg. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 15 (2d8 + 6) slashing damage.

Force Beam. The retriever targets one creature it can see within 60 feet of it. The target must make a DC 16 Dexterity saving throw, taking 27 (5d10) force damage on a failed save, or half as much damage on a successful one.

Paralyzing Beam (Recharge 5-6). The retriever targets one creature it can see within 60 feet of it. The target must succeed on a DC 18 Constitution saving throw or be paralyzed for 1 minute. The paralyzed target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If the paralyzed creature is Medium or smaller, the retriever can pick it up as part of the retriever’s move and walk or climb with it at full speed.

Discharge. When the retriever takes direct damage from a spell, it can use its reaction to cast eldritch blast (3 beams, +5 to hit). 

The retriever 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.  It regains spent legendary actions at the start of its turn.

Subdue. The retriever magically creates a circle of spectral blades that sweep around it. Each creature within 10 feet of the retriever must succeed a DC 16 Dexterity saving throw or take 5d6 force damage, taking half damage on a success.

Observe. The retriever makes a Wisdom (Perception) check with advantage.

Advance (Costs 2 actions). The retriever moves up to its speed.  It can move through the spaces of hostile creatures while doing so.  The retriever can make a foreleg attack against any number of creatures whose spaces it moves through, and the attack deals an additional 1d8 force damage.