Corpse High Guard: Guardian

Crusher. The guardian gains the following abilities.
Enhanced Critical. When the guardian scores a Critical Hit that deals Bludgeoning damage to a creature, attack rolls against that creature have advantage until the start of the guardian's next turn.
Push. Once per turn, when the guardian hits a creature with an attack that deals Bludgeoning damage, the guardian can move it 5 feet to an unoccupied space if the target is no more than one size larger than the guardian. 

Heavy Armor Master. When the guardian is hit by an attack, any Bludgeoning, Piercing, and Slashing damage dealt by that attack is reduced by 4.

Reactive. The guardian can take a second reaction every round. This reaction can only be used to make an Opportunity Attack.

Sentinel. The guardian gains the following abilities.
Guardian. Immediately after a creature within 10 feet of the guard takes the Disengage action or hits a target other than it with an attack, it can make an Opportunity Attack against that creature.
Halt. When the guardian hits a creature with an Opportunity Attack, the creature's speed becomes 0 for the rest of the current turn.

Undead Fortitude. If damage reduces the guardian to 0 Hit Points, it makes a Constitution saving throw (DC 5 plus the damage taken) unless the damage is Radiant or from a Critical Hit. On a successful save, the guardian drops to 1 Hit Point instead.

Multiattack. The guardian can make two attacks with its Warhammer. It can replace one of its attacks with Goading Strike.

Goading Strike. Melee Attack Roll: +9, reach 10 ft. Hit: 10 (1d8+5) Bludgeoning damage plus 4 (1d6) Necrotic damage. The target will have Disadvantage on attack rolls against other creatures besides the guardian.

Warhammer. Melee Attack Roll: +9, reach 10 ft. Hit: 15 (2d8+5) Bludgeoning damage plus 8 (2d6) Necrotic damage.

Rally (4/day). The guardian chooses a number of allies within a 30-foot emanation originating from itself, up to a number of allies equal to its Constitution modifier. Each of those allies gains 1d8+9 temporary Hit Points.

Second Wind (2/day). The guardian has a limited well of stamina it can draw on. As a Bonus Action, it can use it to regain Hit Points equal to 1d12+9. 

Bulwark Blow. Trigger: A creature hits the guardian with a melee attack roll. Response: The guardian makes a melee attack roll against the creature, taking 11 (2d4+5) Bludgeoning damage and knocking them Prone on a hit.

Interception. Trigger: A creature targets another creature within 10 feet of the guardian with an attack roll. Response: The guardian reduces the damage by 1d10+4.