Multiattack

The vampiric knight makes two impaling longsword attacks.

Impaling Longsword

Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 9 (1d8 + 5) slashing damage, or 10 (1d10 + 5) slashing damage if used with two hands, plus 9 (2d8) necrotic damage. The vampiric knight impales the target on its longsword, grappling the target if it is a Medium or smaller creature (escape DC 17). Until the grapple ends, the target is restrained, takes 9 (2d8) necrotic damage at the start of each of its turns, and the vampiric knight can't make longsword attacks against other targets.

Channel Corruption (Recharge 5-6)

Each living creature within 20 feet of the vampiric knight must make a DC 17 Constitution saving throw, taking 42 (12d6) necrotic damage on a failed save, or half as much damage on a successful one.