Infernal Knight

Faultless Tracker. If the infernal knight is given a quarry by its lord, the knight knows the direction and distance to its quarry as long as the two of them are on the same plane of existence.

Magic Resistance. The infernal knight has advantage on saving throws against spells and other magical effects.

Magic Weapons. The infernal knight's weapon attacks are magical.

Regeneration. The infernal knight regains 10 hp at the start of its turn if it has at least 1 hp.

Multiattack. The infernal knight makes two melee attacks or uses its Hellfire Bolt twice. It can replace one attack with Reave Soul.

Greatsword. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: 14 (2d6 + 7) slashing damage plus 17 (5d6) necrotic damage. A creature hit by the sword must succeed on a DC 18 Constitution saving throw or suffer disadvantage on attack rolls and ability checks based on Strength or Dexterity until the end of its next turn.

Hellfire Bolt. Ranged Spell Attack: +10 to hit, range 120 ft., one target. Hit: 10 (3d6) fire damage plus 17 (5d6) necrotic damage. A creature hit must succeed on a DC 18 Constitution saving throw or its hp maximum is reduced by an amount equal to the necrotic damage taken. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hp maximum to 0.

Reave Soul (Recharge 5-6). The infernal knight targets a creature with 0 hp that it can see within 60 feet of it. If the creature is alive, it must succeed on a DC 18 Constitution saving throw or die and have its soul drawn into the infernal knight's greatsword. If the creature is dead and has been dead for less than 1 minute, its soul is automatically captured. When the infernal knight captures a soul, it regains 30 hp, and Reave Soul recharges at the start of its next turn. While a creature's soul is trapped, that creature can't be returned to life by any means short of a wish spell. A banishment spell targeted at the greatsword forces the infernal knight to make a Charisma saving throw against the spell. On a failed save, any souls trapped in the blade are released instead of the spell's normal effect. Trapped souls are also released when the infernal knight dies.

Planar Rift (1/Day). The infernal knight magically tears a rift in the fabric of the multiverse. The rift is a portal to a plane of the infernal knight's choice. The portal remains open for 1 hour, during which time any creature can pass through it, moving from one plane to the other. A dispel magic spell targeting the rift can destroy it if the caster succeeds on a DC 18 spellcasting ability check.