Undead Forge Knight

Undead Fortitude. If damage reduces the undead Forge Knight to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the undead Forge Knight drops to 1 hit point instead.

Inescapable. When undead Forge Knight hits a creature with an opportunity attack, it must make a DC 14 Dexterity saving throw or have its speed reduced to 0 for the rest of the turn.

Multiattack. The undead Forge Knight makes two Forge Hammer attacks, only one of which can be Draining Grasp. It can also replace any of the attacks with Empower Hammer.

Forge Hammer. Melee Weapon Attack: +7 to hit, reach 5 ft., 1 target. Hit: 12 (2d8 + 4) magical bludgeoning damage. 

Draining Grasp. Melee Spell Attack: +6 to hit, reach 5 ft., 1 target. Hit: 16 (4d8) necrotic damage, undead Forge Knight is healed for half that amount and the targets hit point maximum is reduced by that amount until next short or long rest, or until removed by Lesser Restoration or similar spell.

Empower Hammer. For 1 minute, the undead Forge Knights Forge Hammer and Mountain Crack deal an additional (4)1d8 fire damage or 4(1d8) thunder damage, it's choice. The range of Mountain Crack is also increased by 5 feet.

Mountain Crack (Recharge 6). The undead Forge Knight slams the ground in front of him, each creature in a 30 feet line in front of him must succeed on a DC 16 Dexterity saving throw or take 8 (2d8) fire damage, 8 (2d8) thunder damage, and be knocked prone. On success, it only takes half damage and is not knocked prone.

Forge Channel. When subjected to fire or thunder damage, the undead Forge Knight can ignore the damage and make a ranged spell attack(+7 to hit) against a creature or object within 15 feet of it, dealing the original damage to it instead.