Automaton

Shutdown. If targeted by dispell magic the automaton must succeed on a Constitution saving throw against the caster’s spell save DC or fall unconcious for 1 minute or until it takes any damage.

Constructed Resilience. The automaton is immune to any spell or effect that would alter its form. Additionally, the automaton is immune to disease and magic can’t put it to sleep.

Electrical machine. The automaton is immune to lightning damage. Instead whenever the automaton takes lightning damage it gains a charge for every 10 points of damage. If the automaton can sea the source of the lightning it will choose to fail any checks to resist it.
Whenever the automaton gains his fifth charge it becomes overcharged and will explode, every creature within 5ft must make a DC12 dex save, taking 5D6 on a failed save or half as mush on a succesfull one.  

Blueprints. The automaton is made out of a blueprint, you can make an intelligence check to discover vulnerable points lowering its AC with 1 for every 5 points on the intelligence check(19=-3; 3=-0).

Multiattack. The automaton makes two slam attacks or one slam attack and a discharge attack.

Slam. Melee Weapon Attack: +5 to hit, reach 5 ft., 1 target. Hit: 6 (1d6 + 3) [Bludgeoning] damage.

Discharge(cost 1 charge). The automaton uses its electricity to defend itself. All creatures within 5ft must make a DC14 constitution save, taking 1D8+Nr of charges Lightning damage on a failed save.

Allarm. As a bonus action the automaton starts making a loud noise, warning other atomatons for intruders. The sound can be heard in a 120ft radius.