Killing Device

Breakable.  When struck with Bludgeoning damage parts of the construct can be smashed off. What is removed, and how it affects the construct is up to the DM depending on where and how hard it is hit. IE: Lost AC if armour is removed, or reduced speed if a leg is removed or disadvantage on attack – claw rolls if an arm is lost.

Soul Servant. As long as the Killing Device is being controlled by a Master it uses the master's Int, Wis, and skills. Otherwise if the master stops controlling the Killing Device, the Int and Wis return to default. When not being controlled by a master the Killing Device makes attack rolls with disadvantage, but gains +2 to all damage rolls.

Sweet Release. On death, roll a 1d2, on a 1 the soul controlling the Killing Device is released as a CR 4 Ghost, on a 2 every creature within a 15ft radius of the creature takes 2d6 phycic damage, as the soul lashes out before fading.
If a Ghost is released roll a 1d6, below 3 the ghost is hostile to the party, on a 4 or 5 the ghost ignores the party, on a 6 the Ghost joins the party to hunt its master.

Evasion. When a situation allows the Killing Device to take half damage on a Dex saving throw the Killing Device takes no damage on a successful roll and half damage on a failed roll. 

Nimble Escape. Can Disengage or hide as a bonus action,and doesn't trigger reactions.

Charge.  If the Killing Device moves at least 20ft in a straight line towards a target, and then hits a Ram attack on the same turn the target takes an extra (2d6) damage. If the target is a creature it must succeed on a DC 14 strength saving throw or be stunned for one turn.

Electrified Charge. When the Killing Device is hit with Lightning damage it moves up to half of its total speed towards the attacker via the most direct route, and makes a claw attack with disadvantage. On hit the target receives half of the triggering damage in lightning damage. 
The Killing Device moves regardless of whether the movement triggers reactions or Attacks of Opportunity.

Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., 1 target. Hit:  (2d8 + 6) [Piercing] damage. 

Claw. Melee Weapon Attack: +7 to hit, reach 5 ft., 1 target. Hit:  (2d6 + 5) [Slashing] damage. 

Tail. Melee Weapon Attack: +6 to hit, reach 10 ft., 1 target. Hit:  (2d5 + 6) [Bludgeoning] damage. 
On hit the target and one creature adjacent to the target must make a DC 13 Strength  save or be knocked prone. 

Ram. Melee Weapon Attack: +5 to hit, reach 5 ft., # target. Hit:  (2d3 +3) [Bludgeoning] damage. 
On hit the target must make a DC 13 Strength  save or be knocked prone. 

Reconstruct. As an action the Killing Device can grab, and re-attach parts lost to Bludgeoning damage. Does not regain HP, but regains the use of lost parts.  IE. Re-attaching armour would increase the Killing Device's AC.

Multi-Attack.  When below 70 HP the Killing Device can Ram a target and then make either a Claw, and Bite attack, or a Tail attack. 

Frightful Presence.  Each creature of the Device's choice that is within 120 feet of it and aware of its presence must succeed on a DC 16 Wisdom saving throw or become Frightened for 5 rounds. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the Device's Frightful Presence for the next 24 hours.

Defensive Stance.  As an action the Killing Device can prepare to take a second reaction. 

Combat Armour. When the Killing device is below 70 HP whenever it takes damage from any non-magical source, it curls up into a ball. Doing this increases its AC by 2 however it all attacks against it have advantage, and it can not move.  At the start of its next turn it expends half of its movement points to stand up.  

Hasty Retreat. When the Killing Device takes damage from any non-magical source more than 5ft away, it runs 10ft, pushing any creature smaller than it out of its way.  Any creature moved like this must make a DC 14 STR save or take 2d6 bludgeoning damage. 

Lightning Reflexes. When the Killing Device takes damage from any non-magical source within 5 ft the attacker must make a DC 14 Dex save or the Killing Device can make a Tail attack against it.