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.