Corner-hound

Mucus Membrane: Corner-hounds are coated in a layer of semi transparent blue slime. When a creature hits a Corner-hound with a melee attack, they take 2d4 acid damage.

Relentless Hunters: Corner-hounds are relentless in their hunts. If a Corner-hound is slain, it returns to life at 1 hit point in 1 hour.

Advanced Constitution: Corner-hounds have enhanced healing capabilities. At the start of their turn, if a Corner-hound has at least 1 hit point they regain 5 HP. 

Inter-planar Trackers: Corner-hounds are specialized hunters, capable of tracking their quarries through the planes. A Corner-hound knows the plane, distance, and direction of their quarry no matter where they are in the planes. 

Mind-Boggling Visage: Corner-hounds are beings beyond the normal realms of existence, and being exposed to one takes a toll. For every minute a creature spends in the presence of the Corner hound, they must make a DC 17 Intelligence saving through or gain one level of insanity. 

Multiattack: A Corner-hound makes two attacks with it's claws or uses it's Infrasonic Howl.

Claws: +5 to hit, 5ft reach. On hit, the creature takes 2d4 slashing and 2d4 psychic damage.

Infrasonic Howl (Recharge 5-6): The Corner-hound released an infrasonic howl, howling in an increasingly lower pitch. All creatures that can hear within 30 feet of the Corner-hound must make a Constitution saving throw of a DC 14 or only be under the effect of the Slow spell until the Corner-hounds next turn. 

Traverse via Corners: A Corner-hound can use it's action to teleport within 30ft of it's quarry. This ability can only work if the hound enters and exits from an object with a corner of between 80 and 100 degrees.