AUREX, the Purge-Warden (Mecha-Kraken)

Legendary Resistance (6/Day, or 8/Day While the Lab Alarm Is Active). If AUREX fails a saving throw, it can choose to succeed instead.

Magic Resistance. AUREX has Advantage on saving throws against spells and other magical effects.

Immutable Form. AUREX can't be transformed by any means.

Fire Absorption. AUREX takes no fire damage; instead it regains hit points equal to the fire damage dealt.

Siege Monster. AUREX deals double damage to objects and structures.

Regeneration. AUREX regains 30 hit points at the start of each of its turns if it has at least 1 hit point and the Lab Alarm is active.

Reconstitution. When AUREX drops to 0 hit points, it doesn't die. Its body scatters into a Swarm of Golden Sentinels and it becomes Incapacitated. At the start of its next turn it reforms in an empty space within 30 feet with 200 hit points, unless a Shutdown Condition has been met. While reformed this way, its Golden Core is exposed until the end of that turn.

Contamination Protocol. AUREX exists to purge living creatures. Each round it moves toward and concentrates its attacks on the group of living creatures it judges most contaminating. It ignores Constructs and Undead unless they damage it.

Multiattack. AUREX makes two Golden Tentacle attacks and uses Purge Beam (or Unerring Purge against a creature it is grappling).

Golden Tentacle. Melee Attack Roll: +18 to hit, reach 60 ft., one target. Hit: 24 (4d6 + 10) bludgeoning damage plus 10 (3d6) radiant damage, and the target has the Grappled condition (escape DC 21) and is Restrained until the grapple ends. AUREX has ten tentacles.

Purge Beam. Dexterity Saving Throw: DC 21, one creature AUREX can see within 120 ft. Failure: 55 (10d10) radiant damage, and the target can't regain hit points until the start of AUREX's next turn. Success: Half damage.

Unerring Purge. AUREX targets one creature it is grappling; that creature automatically takes 55 (10d10) force damage. If this reduces the target to 0 hit points, it is disintegrated into golden dust.

Contamination Purge (Recharge 5–6). Each creature in a 60-foot Emanation originating from AUREX makes a DC 21 Dexterity saving throw. Failure: 45 (10d8) lightning damage plus 22 (4d10) radiant damage. Success: Half damage.

Containment Field. Dexterity Saving Throw: DC 21, one creature AUREX is grappling (up to 4 contained at once). Failure: The target is sealed in a golden stasis pod: Restrained, has Total Cover from outside effects, and takes 27 (6d8) force damage at the start of each of its turns. If AUREX takes 50+ damage in one turn from a creature inside a pod, it makes a DC 21 Constitution saving throw at the end of that turn or releases all contained creatures (each falls Prone within 10 ft.).

Phase Drift (Dimension Door; Recharge 6). AUREX teleports up to 500 feet to a space it can see or describe, trailing a comet-trail of golden motes. (Recharge 6 ≈ a 5–6 turn cooldown; or track a fixed 5-round cooldown by hand.)

Warden's Ward (Recharges Each Round). When AUREX takes damage, it reduces that damage by 30.

AUREX can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. AUREX regains spent legendary actions at the start of its turn.

Storm Bolt (Costs 1 Action). Dexterity Saving Throw: DC 21, one creature AUREX can see within 120 ft. Failure: 33 (6d10) lightning damage. Success: Half damage.

Sentinel Scatter (Costs 1 Action). A Swarm of Golden Sentinels appears in an empty space within 60 feet of AUREX; or AUREX moves up to half its Speed and makes one Golden Tentacle attack.

Purge Sweep (Costs 2 Actions). AUREX fires a 90-foot-long, 5-foot-wide line of golden light. Each creature in the line makes a DC 21 Dexterity saving throw, taking 44 (8d10) radiant damage on a failure, or half as much on a success.

Lockdown Field (Costs 2 Actions, Recharge 6). AUREX raises a 20-foot cube of golden force (as the Wall of Force spell, shaped as a closed box) around a point it can see within 90 feet, trapping the creatures inside.