I liked the backstory of this guy, very creative. Never saw combat – characters bypassed him. By now you should notice a running theme in my particular group – they are very focused on doing the main mission/quest and will bypass what they can in terms of combat by bypassing or talking their way through. I took the Automata from The Choking Tower and added some ghostly traits.
HAUNTED CASTELLAX-CLASS BATTLE AUTOMATA
Large construct (robot), neutral evil
Armour Class 18 (skymetal armour plating, ghostly protoplasm)
Hit Points 214 (18d12 +40 plus 75 hp force field)
Speed 50’ ft.
Str 29 (+9) Dex 14 (+2) Con 20 (+5)
Int 10 (+0) Wis 18 (+4) Cha 10 (+0)
Saving Throws Str +14, Con +10, Wis +9, Cha +8
Skills Cybertheurgy +8, Athletics+ 14, History +8, Perception +9
Damage Resistances bludgeoning, piercing, and slashing from nonmagical weapons that aren’t adamantine.
Damage Immunities force, necrotic, poison, and psychic
Condition Immunities charmed, frightened, paralyzed, petrified, poisoned, and stunned
Senses Darkvision 60 ft., passive Perception 19
Challenge 15 (10,000 XP)
Gift of the Machine God. As a creature of technology a battle automata gains many damage and condition resistances and immunities.
Combat Hardened. The gearsman’s advanced robotic frame provides a measure of protection against attacks. The Mk II has a damage threshold of 10. It reduces all damage per attack by 10.
Haunted. The unquiet spirit of the gnome druid Jarin Sogron haunts this robot. As a result, the robot takes damage from positive energy as if it were an undead creature and can be detected by detect undead.
Infested. This robot is infested with yellow mold. Every round the robot takes an action or takes damage, the mold fills a 10-foot radius around the robot with poison spores (DMG pg. 105). Sunlight renders the mold dormant, and once the robot takes any amount of fire damage, the mold growing on it is burned away.
Infrasonic Field. Castellax automata constantly emit powerful sound waves at a frequency below normal hearing. These vibrations can debilitate living beings within melee range. Such creatures must succeed at DC 17 Constitution save or fall prone and be nauseated until a save cancels. A warden robot can suppress or resume this ability as a free action.
Reactive Armour. A Castellax automata is covered in a special shell that reacts explosively to powerful strikes. Due to this covering, a warden robot gains light fortification (25% chance to negate critical hits and sneak attacks). When the armor negates a critical hit from a melee weapon, the attacker takes an amount of fire damage equal to half of the damage dealt to the robot. Attackers using reach or ranged weapons avoid this damage.
Resilient. Castellax automata receive a +3 racial bonus on all saving throws. They also hardened to resist lightning attacks eliminating the robots weakness to lightning.
Spotlight. As a bonus action, a Castellax automata can target a single creature with a beam of strobing light that functions as a gaze attack with a range of 60 feet. A creature subject to this attack must succeed at a DC 17 Constitution save or become blinded. Creatures with light blindness or light sensitivity take disadvantage on the saving throw.
Multiattack. Castellax automata makes two attacks of its choice from boltor and shock charger fists.
Shock charger fist. Melee Weapon Attack: +14 to hit, reach 10ft., one target.
Hit: 30 (6d6 + 9) lightning damage.
Boltor. Ranged Weapon Attack: +14 to hit, range 60/240 ft., one target.
Hit: 35 (4d12 + 9) explosive damage.
Mauler Pattern Bolt Cannon (Recharge 5-6). The Castellax can fire a barrage of boltor fire at a specific point it can see within 500 feet of it. Each creature within 10 (2 creatures) feet of that point must make a DC 17 Dexterity saving throw, taking 54 (12d8) explosive (fire and force) damage on a failed save, or half as much damage on a successful one.
During Combat The robot follows Jarin’s impulses and has assumed the druid’s alignment, and as it attacks it proclaims the PCs to be fit only for fertilizer. The robot is entangled on the first round of the fight, but automatically rips free from its roots at the start of the second round—it doesn’t pursue foes from this room. If the positioning is right, the robot tries to bull rush foes over the edge of the pit into area G4.
Morale The haunted robot fights until destroyed.