Here are more traits for your biomechanical animals inspired by Horizon Zero Dawn. Use the guidelines in the DMG or your gut feeling as a DM to determine how much of a CR increase, if any, this adds to the base monster.
Corrupted Machine. Machines are infected with a virus that is both digital and physical in nature. They burn red hot and give off poisonous heat. This makes them both more ferocious but also more susceptible to certain attacks. A corrupted machine gains the following benefits:
- The robot’s hit points are maximised (included).
- The robot gains vulnerability to fire damage.
- Gamma Aura. Any creature that makes a melee attack within 5 feet of the robot suffers 3 (1d6) poison damage and gains the poisoned state on a failed DC (8 + Proficiency bonus + CON) Constitution saving throw.
- Berserk. Once the robot is reduced to half its original hit points it flies into a berserk rage. It has advantage on all attacks and does an additional +[Profecicney bonus) in damage on all its melee attacks. Attackers have advantage on attacks against the robot while it is berserk. It remains berserk until it is repaired back up to full.
Nano Advantage (1/Day). A robot may gain +1d4 on one d20 roll. The robot may wait to see the result of the roll before deciding to use this ability or not. It may also be used as a reaction.
Camouflage. A robot’s outer shell contains colour-shifting polymers that allow the construct to blend into any background. Though not truly invisible, they are hard to pinpoint. The robot has advantage on Dexterity (Stealth) checks.
Infrasound Aura. The robot constantly emit powerful sound waves at a frequency below normal hearing. These vibrations can debilitate living beings within range. Living creatures within 20 feet of the robot must succeed at DC (8+Prof+CON) Constitution save or fall prone and be nauseated until a save cancels. The robot can suppress or resume this ability as a free action. CR +1
Reactive Armour. The robot is covered in a special shell that reacts explosively to powerful strikes. Due to this covering, robot gains a 25% chance to negate critical hits and sneak attacks. When the armour negates a critical hit or sneak attack 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. The robot receives a +3 bonus on all saving throws. They are also hardened to resist lightning attacks eliminating the robots normal damage vulnerabilities to this energy type.
Spotlight. As a bonus action, the robot 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 (8+Prof+CON) Constitution save or become blinded. Creatures with light blindness or light sensitivity have disadvantage on the saving throw.
Concussion Grenade (Recharge on a 6). The robot throws/launches a concussion grenade at a point up to 60 feet away. The robot rolls a d20; on a 1 the grenade misfires and detonates immediately. Each creature within 20 feet of the exploding grenade must make a DC 15 Dexterity saving throw, taking 17 (5d6) bludgeoning damage on a failed save, or half as much damage on a successful one.
Shock Fist. One of the robot’s melee attacks damage die is increased by one and it now does lightining damage.
Berserk Flailing. When a robot becomes bloodied it immediately uses one of its melee attacks against each adjacent enemy.