Axiomatic Mind. The duodrone can’t be compelled to act in a manner contrary to its nature or its instructions.

Disintegration. If the duodrone dies, its body disintegrates into dust, leaving behind its weapons and anything else it was carrying.

Multi-Spectrum Sensors (120 ft.). The duodronecan see normally in darkness and magical darkness, can see invisible objects and creatures, and can automatically detect visual illusions and succeed on saving throws against them. The duodrone can see heat and other forms of radiation outside of a humanoid's visible spectrum. The duodrone can also "see" sound waves and vibrations. The duodrone can see through most barriers and target creatures through even complete cover (treat cover as 1 level lower than normal for the purpose of attack rolls) but is blocked by 1 foot of stone, 1 inch of common metal, a thin sheet of lead, or 3 feet of wood or dirt. Beyond 120 ft., the duodrone can see as well as an average humanoid.

Magic Resistance. The duodrone has advantage on saving throws against spells and other magical effects.

Field Promotion. When a Modron is destroyed or promoted, the nearest Modron of the next rank down is immediately compelled to upgrade itself and fill the gap. The Modron begins upgrading at the start of its next turn, and completes 1 round of upgrades at the start of its next turn after that. The upgrading process takes 1 round for each rank above monodrone that the Modron is upgrading into. During this process, the Modron is incapacitated and keeps all of its current stats and conditions until the upgrade is complete. Once the upgrade is complete, the Modron emerges in its upgraded form with full hit points and no conditions.

Spin Cycle. The duodrone must land and start spinning before it can use any of the Actions below. It takes an Action to start spinning and an Action to stop. The duodrone can't fly while spinning, but gains +2 AC.

Multiattack. The duodrone makes two fist attacks or two charged javelin attacks.

Fist. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 1d6 bludgeoning damage.

Charged Javelin. Melee or Ranged Weapon Attack: +4 to hit, reach 5 ft. or range 30/120 ft., one target. Hit: 1d6 + 1 piercing damage plus 1d4 lightning damage. A duodrone can also target objects and terrain with its javelin attacks. Treat unmoving, defenseless targets like this as having an AC of 10. When the duodrone hits a target with its charged javelins, those javelins are stuck to that target until they are removed. Charged Javelins are removed when they are hit with an attack (AC 12), or pulled out with an Action using a DC 11 Strength (Athletics) check. As a Bonus Action, the duodrone creates two charged javelins in its hands, one positive and one negative. As a Bonus Action, the duodrone can ignite two oppositely charged javelins it's holding. Ignited javelins deal an additional 1d6 fire damage

Plasmatic Arc (must be holding a positive and a negative javelin). A target creature within 5 feet must make a DC 14 Dexterity saving throw. On a failure, the target takes 4d6 lightning damage. On a success, it takes half that. The duodrone can also use this ability to target any point on the boundary of a magnetic control field. If it does so, all creatures within the control field are subject to the Plasmatic Arc's effect.

Magnetic Control Field. When a charged javelin is planted, it sends a wall of electricity (5 ft. high) to any charged javelin of the opposite charge within 30 feet. Any creature caught in this wall (including a creature with one or more javelins stuck in it) takes 1d6 lightning damage for every charged javelin connected to this wall. The line remains until one of the javelins that forms it is removed, and any creature that starts its turn in the wall or enters it takes 1d6 lightning damage per connected javelin in the wall. If walls of lighting between javelins fully enclose a space, that space becomes a magnetic control field for the duodrone (extending 10 feet into the air above). When attacking a target within a Magnetic Control Field with charged javelins, the duodrone ignores the effects of cover and gains advantage on attack rollls.

Magnetic Grasp. If a creature or object weighing no more than 1,000 lbs. is within a magnetic control field, and is mostly made of metal or is wearing armor with large amounts of metal (all medium and heavy armor other than hide), the duodrone can attempt to grip that creature or object using Magnetic Grasp (it must be holding a positive and negative javelin at all times while grasping a creature). The target must succeed on a DC 14 Strength saving throw or else be restrained. If you target a metal object being carried by a creature, that creature may make a DC 14 Strength saving throw to try to hold onto it. The duodrone may hold any number of targets in its Magnetic Grasp (though each target requires an Action to grasp), and may continue to hold them until they escape or are freed. Creatures may attempt to free themselves with a DC 14 Strength saving throw at the end of their turn, or to free adjacent creatures or objects as an Action with a DC 14 Strength (Athletics) check. As a Bonus Action, the duodrone can move a grasped object or creature up to 30 feet in any direction, as well as force it to fall prone, as long as the target stays within the control field. 

Emergency Data Transfer. If the Modron is destroyed by a target it can see, it can transmit all of its knowledge to another Modron within 120 feet just before being destroyed. If it chooses a Modron of lower rank,  It can also give a final simple order to that Modron, which the Modron will attempt to fulfill before doing anything else, including beginning a promotion upgrade. If it chooses Modron of one rank lower, it can select that Modron as the one who will be promoted to fill its rank, even if it isn't the closest.