Automatic Promotion. When Primus is slain, the highest-ranking secundus is immediately promoted to become the new Primus.

Axiomatic Mind. Primus can't be compelled to act in a manner contrary to its nature or its instructions.

Colossal. Primus is immune to forced movement, does not provoke opportunity attacks and cannot make opportunity attacks. 

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

Immutable Existence. Primus is immune to any spell or effect that would alter its form or send it to another plane of existence.

Innate Spellcasting. Primus's spellcasting ability is Intelligence (spell save DC 26). It can innately cast the following spells, requiring no components:

• at will: detect thoughts, dispel magic, heat metal, mending, pulse wave, silvery barbs
• 3/day: antilife shell, gravity sinkhole, hold monster, slow, temporal shunt, wall of force
• 1/day: power word kill, time stop, wish

Law of Truth. Creatures within 120 feet of Primus can't speak deliberate lies.

Legendary Resistance (5/Day). If Primus fails a saving throw, it can choose to succeed instead.

Limited Magic Immunity. Unless it wishes to be affected, Primus is immune to spells of 6th level or lower that target it or include it in their area of effect. Primus has advantage on saving throws against all other spells and magical effects.

Scales of Equilibrium. Primus's arms represent a pair of scales, balancing light against dark. As long as these scales are in balance, all dice rolls of any kind made by Primus always yield the average result, Primus can use both its light and dark attacks and it projects an aura of law and order within 120 feet of it, which causes the following effects:

• The area within 120 feet of Primus is filled with dim light and any modron within that area cannot be charmed or frightened.
• All damage rolls made by all creatures within 120 feet of Primus always yield the average result.
• No roll made by a creature within 120 feet of Primus can be affected by advantage or disadvantage.

If Primus takes at least 60 radiant or necrotic damage in a single turn, the scales are thrown out of balance, tipping towards either light if triggered by radiant damage or dark if triggered by necrotic damage. When this happens, all creatures of Primus's choice within 120 feet of Primus, must make a DC 26 Wisdom saving throw or be frightened until balance is restored. A frightened creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

In addition, while in this state, Primus no longer projects an aura, is immune to the triggering damage type, cannot use its bonus actions and some of its other abilities change accordingly.

Once the scales have been thrown out of balance, they remain in that state for three rounds or until Primus takes at least 60 damage of the opposite type.

Multiattack. Primus makes an attack with its Gyro Slam once against each creature of its choice within 20 feet of it while rotating around its own axis and then uses dark and light once each. If a creature got hit by both light and dark on the same turn, Primus uses its Enforce Balance on it.
if Primus's scales are out of balance, it can only use either light or dark depending on the triggering damage type.

Light. Ranged Spell Attack: automatic hit, range 120 ft., one target. Hit: 30 radiant damage.
If Primus's scales are out of balance, this attack requires an attack roll with a +18 to hit, deals 54 (12d8) radiant damage and the target must make a DC 26 Constitution saving throw or be blinded until the end of its next turn.

Dark. Ranged Spell Attack: automatic hit, range 120 ft., one target. Hit: 30 necrotic damage.
If Primus's scales are out of balance, this attack requires an attack roll with a +18 to hit, deals 54 (12d8) necrotic damage and the target must make a DC 26 Constitution saving throw or have its maximum HP reduced by an amount equal to the damage taken.

Gyro Slam. Melee Weapon Attack: automatic hit, reach 20 ft., one target. Hit: 60 bludgeoning damage and the target is pushed 10 feet away from Primus and knocked prone if it is huge or smaller.
If Primus's scales are out of balance, this attack requires an attack roll with a +18 to hit and deals 54 (12d8) bludgeoning damage.

Enforce Balance. Primus targets a creature within 120 feet of it that got hit by both light and dark on the same turn. The target must succeed on a DC 26 Charisma saving throw, or it cannot use reactions or bonus actions and all its dice rolls always yield the average result until the end of its next turn.

Pulse of Equilibrium (Recharge 5–6). Primus's spheres of light and dark begin to pulse and emit a wave of magical energy with a 120-feet radius centered on Primus. The effect depends on the state Primus's scales of equilibrium are in:

• Balance: All creatures of Primus's choice within the area take 30 radiant and 30 necrotic damage, all summoned creatures within the area are returned to their planes of origin and spells cast to summon them end, and all modrons within the area regain 60 hit points.
• Out of balance, dark: All creatures of Primus's choice within the area must make a DC 26 Constitution saving throw, taking 90 (20d8) necrotic damage on a failure or half as much on a success and the area becomes filled with magical darkness until the start of Primus's next turn. In addition, until the start of Primus''s next turn, any modron within the area deals an additional 1d8 necrotic damage with its attacks.
• Out of balance, light: All creatures of Primus's choice within the area must make a DC 26 Constitution saving throw, taking 90 (20d8) radiant damage on a failure or half as much on a success and the area becomes filled with bright daylight until the start of Primus's next turn. In addition, on a failed save, a creature gains a level of exhaustion, and until the start of Primus's next turn any modron within the area deals an additional 1d8 radiant damage with its attacks.

Shift Gear. (3/day) The great gears start picking up speed, empowering Primus itself and modrons fighting at its side to restore law and order in the Clockwork Nirvana. Until the start of Primus's next turn, Primus itself and all modrons within 120 feet of Primus gain a +5 bonus to their attack rolls and ability checks and have their movement speed doubled.

Armor of Mechanus (3/day). Gears and other metal clockwork parts manifest as protective armor around Primus and any modron within 120 feet, granting them 60 temporary hit points, and, while these temporary hit points persist, a +5 bonus to their AC and saving throws.

Blood of the Machine. When Primus has less than half its maximum hit points and takes damage from a weapon attack, an oil-like liquid leaks from the wound. Each creature within 20 feet of Primus takes 24 acid damage and must make a DC 26 Dexterity saving throw or fall prone. The area within 20 feet of Primus is considered difficult terrain for creatures other than modrons or Primus itself until the start of Primus's next turn.

Primus can take 5 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. Primus regains spent legendary actions at the start of his turn.

Command Modron Squadron. Primus commands up to 6 modrons it can see within 120 feet of it to use their reactions to make one weapon attack.

Modron March through Space and Time. Primus teleports up to 6 modrons it can see within 120 feet of it to unoccupied spaces within that area.

Scan. Primus learns the location of any hidden creature and the presence of any magic as per the detect magic spell within the Great Modron Cathedral.

Cast a Spell (Costs 2 Actions). Primus casts one of its spells.

Light and Dark (Costs 2 Actions). Primus makes one attack with its light and one attack with its dark. If a creature got hit by both light and dark on the same turn, Primus uses its Enforce Balance on it.
if Primus's scales are out of balance, it can only use either light or dark depending on the triggering damage type.

Clockwork Cavalcade (Costs 3 Actions). Primus summons the following amount of modrons: one pentadrone, two quadrones, three tridrones, four duodrones and five monodrones. The summoned creatures have maximum hit points, appear in unoccupied spaces within 120 feet of Primus and obey its commands. Whenever one of these modrons dies, unless it was a monodrone, a new modron of a lower rank is immediately promoted to fill that spot according to their hierarchy. When Primus uses this legendary action again, only those spots vacated by destroyed or promoted modrons are filled up.