I originally planed to have the UNITY fight staggered as I didn’t expect the characters to rest after they started engaging UNITY directly. Though the fights are not one after the other I consider them one big fight in that context with the UNITY AI directly harassing the players via spells an minions.
- PHASE 1: UNITY AI harasses party as they progress through the ship.
- PHASE 2: UNITY AI + Overlord robot battle
- PHASE 3: UNITY AI + UNITY’s Avatar in the GODMIND.
I was toying with the idea of having a phase 4 where the characters fight a Phase 1 UNITY in the bowels of the shuttle as she launches herself in to space – zero grav battle against a foe without a body. Something similar to System Shock 2. But never really worked it out beyond an idea.
I also put together a list of dialogue for UNITY to use throughout the adventure. Mostly inspired by SHODAN from System Shock 2. But I also poached ideas from the Iron Gods forum thread on the Divinity Drive.
I can’t help but feel the visual design is inspired by (or takes cues from) the Lady of Pain from Sigil and so renamed her appropriately.
UNITY 
“Lady of Pain” Robot
Large construct (robot, aggregate), lawful evil
Armour Class 22 (natural armour)
Hit Points 408 (24d10 + 168)
Speed 50 ft., fly 30 ft.
STR 26 (+8) DEX 14 (+2) CON 24 (+7)
INT 34 (+12) WIS 22 (+6) CHA 26 (+8)
Saving Throws Int +12, Chr +10, Wis +8, Dex +8, Con +13
Skills Cybertheurgy +15, History +9, Perception +14, Persuasion +10
Damage Resistances piercing, and slashing from non-magical weapons that aren’t adamantine.
Damage Immunities bludgeoning, cold, necrotic, poison, and psychic
Condition Immunities charmed, frightened, paralysed, petrified, poisoned, and stunned
Senses truesight 120ft., passive Perception 22
Languages Abyssal, Aklo, Gothic, Aquan, Auran, Celestial, Common, Draconic, Dwarven, Elven, Giant, Gnome, Halfling, Hallit, Ignan, Infernal, Lashunta, Orc, Terran
Challenge 20 (25,000 XP)
Combat Hardened. The overlord robot has reinforced armour that provides a measure of protection against attacks. The overlord has a damage threshold of 10. It reduces all damage by ten.
Impart Mythic Traits. When UNITY inhabits an aggregate, it grants the aggregate a +8 bonus to natural armour and an additional 80 hit points. Aggregates lose all their vulnerabilities and gain the benefits of all of UNITY’s special traits. See UNITY entry for details.
Magic Weapons. The overlord robot’s weapon attacks are magical.
Powerful Attack. The overlord robot may take disadvantage on a melee attack roll and if it hits it does an extra 10 points of the appropriate damage.
ACTIONS
Multiattack. The overlord robot makes four attacks: one with its laser, one with its vorpal axe, and two with its adamantine. She may also use Gravity Bomb when available.
Vicious Adamantine Claw. Melee Weapon Attack: +14 to hit, reach 10 ft., one target.
Hit: 17 (2d8 + 8) piercing damage. A strike from an adamantine claw automatically brings down force fields. In addition on a natural 20 it does an additional 7 damage.
Laser. Ranged Weapon Attack: +14 to hit, range 120 ft., one target.
Hit: 22 (5d6 + 2) radiant damage.
Vorpal Axe. Melee Weapon Attack: +14 to hit, reach 10ft., one target.
Hit: 24 (3d1O + 8) slashing damage. This weapon ignores slashing resistance. On a natural 20 it decapitates the target instantly killing them.
Gravity Bomb (Recharge 5 or 6). The overlord robot can fire a purple bolt of force from its eye. This bolt has a range of 120 feet and explodes on impact, dealing 70 (20d6) force damage to creatures in a 30-foot radius (Dexterity DC 21 half). Creatures that fail this save are also encapsulated in a sphere of force, as Otiluke’s resilient sphere, for 1 minute (PHB pg. 264).
LEGENDAY ACTIONS
UNITY can take 3 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. The dragon regains spent legendary actions at the start of its turn.
Detect. UNITY makes a Wisdom (Perception) check.
Move. The overlord robot takes a move action.
Heavy Ordnance. (Costs 2 Actions). The overlord robot launches a mortar strike cantered on it. Each creature within 15 feet of the overlord robot must succeed on a DC 22 Dexterity saving throw or take 10 (1d6 + 8) bludgeoning damage plus 10 (1d6 + 8) fire damage and be knocked prone. UNITY depends on her resistances, immunities and combat hardness to avoid taking any damage.
TACTICS
Before Combat The overlord robot casts unholy aura on itself.
During Combat The overlord robot relies on its physical attacks rather than spell-like abilities. It uses its gravity bomb as often as possible, as well as Deadly Aim and Power Attack. If foes are hitting it too often, the overlord swaps out the use of Power Attack for Combat Expertise and begins to use Flyby Attacks to try to limit full attacks against itself.
Morale The overlord robot fights until destroyed.